cleanup: Mark unused arguments as unused

This will stop eslint from warning about them, while keeping their
self-documenting benefit.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/627

js/gdm/loginDialog.js

@@ -1241,7 +1241,7 @@ var LoginDialog = GObject.registerClass({
         this._authPrompt.cancel();
     }
 
-    addCharacter(unichar) {
+    addCharacter(_unichar) {
         // Don't allow type ahead at the login screen
     }