mpm_event: follow up to r1666468 and r1666618.

We don't need to return 0 in the compare function, but for debugging purpose
which we could implement later if necessary (in a separate function).
For now, keep the function simple as in 2.4.x to ease backport, and add a
comment about why we never return 0 here.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1674697 13f79535-47bb-0310-9956-ffa450edef68