Remove the link to mbox.apachelabs.org. (Amazingly, that link worked.)

But, we cite everything via the Message-ID to be archive-neutral.
(FWIW, www.apachelabs.org has a feature to search on Message-ID.)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92815 13f79535-47bb-0310-9956-ffa450edef68