mpm_event: follow up to r1666468.

We only need one compare function for add semantic with apr_skiplist_insert()
and unique timers (pointers). It also should work with apr_skiplist_remove()
and apr_skiplist_find(), be they used some day.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1666618 13f79535-47bb-0310-9956-ffa450edef68