mod_proxy: Tone down a warning message that appears when the same backend

website is reverse proxied two or more times into the frontend URL space.
PR: 48947
minfrin's patch


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@962992 13f79535-47bb-0310-9956-ffa450edef68