extensionUtils: Remove userExtensionsDir

Make this less stateful

https://bugzilla.gnome.org/show_bug.cgi?id=677586

js/extensionPrefs/main.js

@@ -268,7 +268,6 @@ function initEnvironment() {
 
 function main(argv) {
     initEnvironment();
-    ExtensionUtils.init();
 
     Gettext.bindtextdomain(Config.GETTEXT_PACKAGE, Config.LOCALEDIR);
     Gettext.textdomain(Config.GETTEXT_PACKAGE);