Remove rigid body ID assignment from Compound

[chrisjonesBSU]

As the simulation engine writers are migrating from mBuild to GMSO, questions came up about how to handle particle-level rigid IDs when adding HOOMD rigidy body support to GMSO.

@CalCraven and I believe rigid body assignment and writing is best handled in GMSO rather than in mBuild (and then converted when going from mbuild --> GMSO).

So, we should remove the rigid_id data structures from the Compound class in mBuild as it won't be used anywhere once #1191 is merged.