Port Bug 1492937 - Make the JS subscript loader load scripts exclusively as UTF-8 (#4600)

aboutNewTabService.js

@@ -163,7 +163,7 @@ AboutNewTabService.prototype = {
           }
 
           for (let script of scripts) {
-            Services.scriptloader.loadSubScript(script, win, "UTF-8"); // Synchronous call
+            Services.scriptloader.loadSubScript(script, win); // Synchronous call
           }
         };
         subject.addEventListener("DOMContentLoaded", onLoaded, {once: true});