Cleanup / revise xacro parameters

Clean up and/or revise the parameters used in xacro files. Some may not be required anymore, some may be due for renaming for consistency (aka fake_hardware vs mock_sensors)

Example to simplify:
```
  <xacro:macro name="rrbot_transmissions_system_position_only" params="name prefix use_gazebo:=^|false use_fake_hardware:=^|true mock_sensor_commands:=^|false slowdown:=2.0">
```