Cleanup more unreferenced MutexGuard parameters on functions.

These parameters are intended to serve as sort of a contract that
you cannot access the functions outside of a mutex.  However, the
entire JIT class cannot be accessed outside of a mutex anyway, and
all methods acquire a lock as soon as they are entered.  Since the
containing class already is not intended to be thread-safe, it only
serves to add code clutter.

llvm-svn: 211071