Clean up left-behind dispatcher handlers when removing RainMachine

[bachya]

Description:

The current implementation of RainMachine leaves behind some active dispatcher handlers when removing the component via the UI. This PR cleans that up (and centralizes that logic in the component, rather than across the platforms).

Related issue (if applicable): N/A

Pull request in home-assistant.io with documentation (if applicable): N/A

Example entry for configuration.yaml (if applicable):

rainmachine:
  ip_address: !secret rainmachine_host
  password: !secret rainmachine_password

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.