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/ui/barLevel.js

@@ -188,7 +188,7 @@ var BarLevel = class {
         return this._maxValue;
     }
 
-    _setCurrentValue(actor, value) {
+    _setCurrentValue(_actor, value) {
         this._value = value;
     }