Improve MQTT test coverage and remove dead code

[emontnemery]

Proposed change

Improve MQTT test coverage and remove dead code

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
• 🥈 Silver
• 🥇 Gold
• 🏆 Platinum

[probot-home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as its been labeled with a integration (mqtt) you are listed as a codeowner for? Thanks!

[emontnemery]

Test coverage without PR:

----------- coverage: platform linux, python 3.7.5-final-0 -----------
Name                                                     Stmts   Miss  Cover
----------------------------------------------------------------------------
homeassistant/components/mqtt/__init__.py                  615     46    93%
homeassistant/components/mqtt/abbreviations.py               2      0   100%
homeassistant/components/mqtt/alarm_control_panel.py       143      1    99%
homeassistant/components/mqtt/binary_sensor.py             124      0   100%
homeassistant/components/mqtt/camera.py                     69      0   100%
homeassistant/components/mqtt/climate.py                   390      1    99%
homeassistant/components/mqtt/config_flow.py                67     21    69%
homeassistant/components/mqtt/const.py                       9      0   100%
homeassistant/components/mqtt/cover.py                     282      8    97%
homeassistant/components/mqtt/debug_info.py                 68      2    97%
homeassistant/components/mqtt/device_automation.py          29      0   100%
homeassistant/components/mqtt/device_tracker.py             33      0   100%
homeassistant/components/mqtt/device_trigger.py            150      2    99%
homeassistant/components/mqtt/discovery.py                  95      2    98%
homeassistant/components/mqtt/fan.py                       201      4    98%
homeassistant/components/mqtt/light/__init__.py             31      0   100%
homeassistant/components/mqtt/light/schema.py                3      0   100%
homeassistant/components/mqtt/light/schema_basic.py        386      0   100%
homeassistant/components/mqtt/light/schema_json.py         271      0   100%
homeassistant/components/mqtt/light/schema_template.py     222      0   100%
homeassistant/components/mqtt/lock.py                      106      0   100%
homeassistant/components/mqtt/models.py                      9      0   100%
homeassistant/components/mqtt/sensor.py                    104      0   100%
homeassistant/components/mqtt/server.py                     37      0   100%
homeassistant/components/mqtt/subscription.py               48      0   100%
homeassistant/components/mqtt/switch.py                    115      0   100%
homeassistant/components/mqtt/vacuum/__init__.py            29      0   100%
homeassistant/components/mqtt/vacuum/schema.py              16      0   100%
homeassistant/components/mqtt/vacuum/schema_legacy.py      231      2    99%
homeassistant/components/mqtt/vacuum/schema_state.py       153      1    99%
----------------------------------------------------------------------------
TOTAL                                                     4038     90    98%

Test coverage with the PR:

----------- coverage: platform linux, python 3.7.5-final-0 -----------
Name                                                     Stmts   Miss  Cover
----------------------------------------------------------------------------
homeassistant/components/mqtt/__init__.py                  615     31    95%
homeassistant/components/mqtt/abbreviations.py               2      0   100%
homeassistant/components/mqtt/alarm_control_panel.py       143      0   100%
homeassistant/components/mqtt/binary_sensor.py             124      0   100%
homeassistant/components/mqtt/camera.py                     69      0   100%
homeassistant/components/mqtt/climate.py                   390      0   100%
homeassistant/components/mqtt/config_flow.py                67     21    69%
homeassistant/components/mqtt/const.py                       9      0   100%
homeassistant/components/mqtt/cover.py                     274      0   100%
homeassistant/components/mqtt/debug_info.py                 68      0   100%
homeassistant/components/mqtt/device_automation.py          29      0   100%
homeassistant/components/mqtt/device_tracker.py             33      0   100%
homeassistant/components/mqtt/device_trigger.py            150      2    99%
homeassistant/components/mqtt/discovery.py                  90      0   100%
homeassistant/components/mqtt/fan.py                       197      0   100%
homeassistant/components/mqtt/light/__init__.py             31      0   100%
homeassistant/components/mqtt/light/schema.py                3      0   100%
homeassistant/components/mqtt/light/schema_basic.py        386      0   100%
homeassistant/components/mqtt/light/schema_json.py         271      0   100%
homeassistant/components/mqtt/light/schema_template.py     222      0   100%
homeassistant/components/mqtt/lock.py                      106      0   100%
homeassistant/components/mqtt/models.py                      9      0   100%
homeassistant/components/mqtt/sensor.py                    104      0   100%
homeassistant/components/mqtt/server.py                     37      0   100%
homeassistant/components/mqtt/subscription.py               48      0   100%
homeassistant/components/mqtt/switch.py                    115      0   100%
homeassistant/components/mqtt/vacuum/__init__.py            29      0   100%
homeassistant/components/mqtt/vacuum/schema.py              16      0   100%
homeassistant/components/mqtt/vacuum/schema_legacy.py      227      0   100%
homeassistant/components/mqtt/vacuum/schema_state.py       151      0   100%
----------------------------------------------------------------------------
TOTAL                                                     4015     54    99%