`rra_controls_file` property in CMCTool may mislead users

[carmichaelong]

A user on the forum thought they had to use the rra_controls_file property for CMC: https://simtk.org/plugins/phpBB/viewtopicPhpbb.php?f=91&t=15181&p=0&start=0&view=&sid=83722852d74c22e92e79772a5e920844

The code seems to indicate that this is not necessary: https://github.com/opensim-org/opensim-core/blob/master/OpenSim/Tools/CMCTool.cpp#L996

We should consider removing this property, or clarifying in the error message that this is not needed for CMC (rather than saying that this isn't implemented yet)