APC extension no longer enabled, remove permission

No idea when it was removed, probably after I22a0978, maybe because
it's allegedly incompatible with recent releases of MediaWiki.
php -r 'phpinfo();' | grep apc can verify whether apc is enabled on
the server.

Change-Id: Ibee1c276b1f7f120ea41c7be6ee2a6c46533a478