Clean up some unnecessary mutex guards.

These were being used as unreferenced parameters to enforce that
the methods must not be called without holding a mutex, but all
of the methods in question were internal, and the methods were
only exposed through an interface whose entire purpose was to
serialize access to these structures, so expecting the methods
to be accessed under a mutex is reasonable enough.

Reviewed by: blaikie

Differential Revision: http://reviews.llvm.org/D4162

llvm-svn: 211054