Remove more occurrences of the unused-mutex-parameter pattern.

This pattern loses some of its usefulness when the mutex type is
statically polymorphic as opposed to runtime polymorphic, as
swapping out the mutex type requires changing a significant number
of function parameters, and templatizing the function parameter
requires the methods to be defined in the headers.

Furthermore, if LLVM is compiled with threads disabled then there
may even be no mutex to acquire anyway, so it should not be up to
individual APIs to know whether or not acquiring a mutex is required
to use those APIs to begin with.  It should be up to the user of the
API.