Remove Freeze bounds from sync::MutexArc

With Rc no longer trying to statically prevent cycles (and thus no
longer using the Freeze bound), it seems appropriate to remove that
restriction from MutexArc as well.
Remove the compile-fail test that's now obsolete
auto merge of #12336 : kballard/rust/mutexarc-no-freeze, r=alexcrichton

With Rc no longer trying to statically prevent cycles (and thus no
longer using the Freeze bound), it seems appropriate to remove that
restriction from MutexArc as well.

Closes #9251.