the mpm-note-child-killed hook was just for use by

ap_{reclaim,relieve}_child_processes; those functions can
invoke the hook directly; the function ap_mpm_note_child_killed()
is useless


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1086953 13f79535-47bb-0310-9956-ffa450edef68