Remove $wgUseTeX

$wgUseTeX is not needed in the extension. For backwards compatibility,
it's also set in /mediawiki/includes/DefaultSettings.php and used in
/mediawiki/includes/EditPage.php only.

Change-Id: Ie5bcabf9e2e7474a4d41d1efcc5072c144640388
Merge "Remove $wgUseTeX"