INSTALL: Don't warn against using PHP "as a CGI plugin"

INSTALL: Don't warn against using PHP "as a CGI plugin"

This note is very old; it was added in r2832 (635388356be70130) and
r3285 (7ab7a50cf928861f). Modern versions of MediaWiki support CGI and
FastCGI installations nearly as well as mod_php installations. In fact,
Wikimedia currently uses HHVM's "fastcgi" server type, and intends to
use php-fpm with PHP 7 (see migration plan in T176370).

For a basic MediaWiki installation using the web installer, the only
difference I know of is the default URLs: /w/index.php?title=$1 for CGI,
as opposed to /w/index.php/$1 for mod_php. This issue is easy to fix by
changing $wgArticlePath, if the web server sets REQUEST_URI. Admins who
want to use short URLs (e.g. /wiki/$1) have to do this anyway.

Also, in img_auth.php and the "img-auth-nopathinfo" message, don't imply
that CGI and FastCGI server configurations generally can't provide path
information or support MediaWiki features that need it.

Change-Id: I89212e9d51f950a58f911083b9e109b69cd4e060