proxy_fcgi: remove FPM-specific logic

Reverts r1780328, r1780329, and their associated followups, which
incorrectly manipulated SCRIPT_NAME by default. All proxy_fcgi.t
regression tests now pass.

PR: 61202

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1800306 13f79535-47bb-0310-9956-ffa450edef68