Get the saved font using the fontSelector defined in mw.webfonts.setup

Also make sure that correct language is passed to the getFont
It was wrong in current version - UI language was passed always
(introduced in a refactoring to avoid code duplication for content
and ui language- commit: 98bb1c200c )

Change-Id: I62860f9d62538915628ad21e5d904e56bdcc9430

resources/js/ext.uls.webfonts.js

@@ -62,10 +62,6 @@
 					font = repository.defaultFont( language );
 				}
 
-				if ( font === 'system' ) {
-					font = null;
-				}
-
 				return font;
 			},
 			exclude: ( function () {