cleanup: Remove some unhelpful unused arguments

Those unused arguments aren't bugs - unbeknownst to eslint, they all
correspond to valid signal parameters - but they don't contribute
anything to clarity, so just remove them anyway.

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

js/ui/background.js

@@ -635,7 +635,7 @@ var Animation = class Animation {
     load(callback) {
         this._show = new GnomeDesktop.BGSlideShow({ file: this.file });
 
-        this._show.load_async(null, (object, result) => {
+        this._show.load_async(null, () => {
             this.loaded = true;
             if (callback)
                 callback();