MB-8214 remove unnecessary mutex notification

ExecutorThread::reschedule() method gets called very
often. This does not need to call mutex notification
at all since the running thread itself calls the method
during its runtime.

Change-Id: I66b0fe60c82c604d173fdab8090bd7461fe2cece
Reviewed-on: http://review.couchbase.org/26721
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Jin Lim <jin@couchbase.com>