Revision 1670953

Fix building with apr trunk by removing unnecessary header includes.

apr_thread_mutex.h is included by apr_allocator.h which is included
by apr_pools.h.  This header is prone to circular references because
it also includes apr_pools.h and despite efforts to prevent this
it's blowing up clang pretty bad with apr trunk.  However, we don't
need it at all because we always have apr_pools.h.

* subversion/include/private/svn_mutex.h,
  subversion/libsvn_fs/fs-loader.c,
  subversion/libsvn_fs_fs/fs.c,
  subversion/libsvn_fs_x/fs.c,
  subversion/libsvn_subr/cache-inprocess.c,
  subversion/libsvn_subr/dso.c,
  subversion/libsvn_subr/pool.c:  Remove apr_thread_mutex.h include.