r5306: removed all the unused mutex functions from mutex.c. When (if?) we

r5306: removed all the unused mutex functions from mutex.c. When (if?) we
decide to reinstate the mutex code for the threads process model, I'd
like to do it a little differently. At least this gets it out of
includes.h for now.
(This used to be commit cfee0fb02e10add22b6c436bdfa95d1a8f5f3def)