Simplify CMakelists of ca_gazebo

[eborghi10]

Description

This is a small PR intended to simplify the CMakeLists.txt file of the ca_gazebo package to make it simpler to understand.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

# Run Gazebo and check that all the plugins are okay
roslaunch ca_gazebo create_empty_world.launch

Test Configuration:

• Real robot
• Gazebo simulation
• Other (please specify)

Checklist:

• My code follows the style guidelines of this project (Travis CI is passing)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes