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/slider.js

@@ -202,7 +202,7 @@ var Slider = class extends BarLevel.BarLevel {
         this.emit('value-changed', this._value);
     }
 
-    _getMinimumIncrement(actor) {
+    _getMinimumIncrement() {
         return 0.1;
     }
 };