Remove the ap_thread_mutex code from all MPM's. This code isn't actually

being called anywhere, and I have compiled dexter, mpmt_pthread, and
prefork without it.  Away it goes.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@84138 13f79535-47bb-0310-9956-ffa450edef68