Remove use of navigator.userLanguage for IE11

Since MediaWiki 1.41 the Internet Explorer 11 has only Basic support:
https://www.mediawiki.org/wiki/Compatibility#Desktop

Change-Id: I4f4fb6532b0fe7aab9a3cb787deb4731d899bb53

resources/js/ext.uls.common.js

@@ -181,10 +181,9 @@
 	 */
 	mw.uls.getBrowserLanguage = function () {
 		// language is the standard property.
-		// userLanguage is only for IE and returns system locale.
 		// Empty string is a fallback in case both are undefined
 		// to avoid runtime error with split().
-		return ( window.navigator.language || window.navigator.userLanguage || '' ).split( '-' )[ 0 ];
+		return ( window.navigator.language || '' ).split( '-' )[ 0 ];
 	};
 
 	mw.uls.getCountryCode = function () {