Remove support for magic_quotes_gpc

Remove support for magic_quotes_gpc

This "feature" of PHP has been disabled by default for quite some
time, deprecated in 5.3 and removed entirely in 5.4.

See <http://www.php.net/manual/en/security.magicquotes.php> for
information on why this practice is bad and how to disable it.

For MediaWiki, the removed todo comment says it all. Doing this
messes with all of the super globals and makes embedding MediaWiki
inside another application problematic. It also increases our
memory footprint for every single request even when the data might
not be used.

Finally, it makes it possible to instantiate new WebRequest objects
pretty much whenever you want, so there's no reason for $wgRequest
to exist anymore.

Bug: 11558
Change-Id: I4bc0392276f454ceb966547c7fbd7c8190ebbc7c