rx: MUTEX_* are no-ops when !RX_ENABLE_LOCKS

rx: MUTEX_* are no-ops when !RX_ENABLE_LOCKS

Remove any occurences of
    # ifdef RX_ENABLE_LOCKS
    MUTEX_ENTER(&mutex)
    # endif

and similar for MUTEX_INIT, MUTEX_DESTROY and MUTEX_EXIT, as all of
these operations are no-ops when RX_ENABLE_LOCKS isn't defined.

Change-Id: Ic0f27b42d56ae4362ff095f1f08a06575c4ca2c1
Reviewed-on: http://gerrit.openafs.org/8293
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>