Remove OpenMM

This code is not functional in release-4-6 branch, and nobody has
plans to fix it. Many things are already out of sync in the duplicated
code. This will get much worse as time passes, so simplifying the few
places that still touch the main code base has slight advantages. And
nobody will feel the need to clean up the duplicated code.

Change-Id: I06c2eb71042fd830fd888fbee1b5e3efcae0379d