Implement mod_disk_cache's remove_url.

(Minor tweaks and comment fixes by Justin.)

Suggested by: Paul Querna, Justin Erenkrantz
Submitted by: Rudiger Plum <ruediger.pluem vodafone.com>
Reviewed by: Justin Erenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@231486 13f79535-47bb-0310-9956-ffa450edef68