cleanup: Remove some unhelpful unused variables in destructuring

We aren't using them, and they don't add much in terms of clarity,
so drop them to fix a couple of eslint errors.

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

js/ui/slider.js

@@ -144,7 +144,7 @@ var Slider = class extends BarLevel.BarLevel {
         } else if (direction == Clutter.ScrollDirection.UP) {
             delta = SLIDER_SCROLL_STEP;
         } else if (direction == Clutter.ScrollDirection.SMOOTH) {
-            let [dx, dy] = event.get_scroll_delta();
+            let [, dy] = event.get_scroll_delta();
             // Even though the slider is horizontal, use dy to match
             // the UP/DOWN above.
             delta = -dy * SLIDER_SCROLL_STEP;