mod_proxy: axe negative "ping" parameter setting and handling.

This used to check for the backend connection readability only (instead of
the full ping/100-continue round-trip), but the case is already handled by
ap_proxy_connect_backend() which is always called.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1729507 13f79535-47bb-0310-9956-ffa450edef68