LanguageNameIndexer: Stop writing to a global

It has no effect anymore. I could not find easy way to achieve what
it is trying to do, so just making the script exit with an error if
custom languages are present.

Change-Id: I693503b4ec46e6ac3eb4a11f4ce065f960f2ec62

data/LanguageNameIndexer.php

@@ -39,7 +39,9 @@ class LanguageNameIndexer extends Maintenance {
 		global $wgExtraLanguageNames;
 
 		// Avoid local configuration leaking to this script
-		$wgExtraLanguageNames = [];
+		if ( $wgExtraLanguageNames !== [] ) {
+			$this->fatalError( 'You have entries in $wgExtraLanguageNames. Needs to be empty for this script.' );
+		}
 
 		$languageNames = [];
 		// Add languages from language-data