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/util.js

@@ -303,7 +303,7 @@ var ShellUserVerifier = class {
             });
     }
 
-    _oVirtUserAuthenticated(token) {
+    _oVirtUserAuthenticated(_token) {
         this._preemptingService = OVIRT_SERVICE_NAME;
         this.emit('ovirt-user-authenticated');
     }