Clean up old way to disable variants (using a special name 'disable')

$wgDisabledVariants is the preferred setting now since 1.16. For some
reason, the 'disable' magic name hasn't been cleaned up completely and
only affects skinning.

Someone may want to run rebuildLanguage.php later because messages.inc
is modified here. I didn't run it because it introduces too many
unrelated edits messing up this patch.

Change-Id: Ifb005e7cc9efd0b4a4f87fd491a71c8d5e1a5748