fileUtils: Delete deleteGFile hack

It is true that delete is a javascript keyword, but that doesn't
prevent it from being used as method name - there are event built-in
types like Map or Set with delete() methods!

So if that hack was ever needed, this hasn't been the case for years
now; just removed the hack now.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/862

js/ui/extensionSystem.js

@@ -41,7 +41,7 @@ var ExtensionManager = class {
         }
 
         GLib.timeout_add_seconds(GLib.PRIORITY_DEFAULT, 60, () => {
-            FileUtils.deleteGFile(disableFile);
+            disableFile.delete(null);
             return GLib.SOURCE_REMOVE;
         });