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/dash.js

@@ -296,7 +296,7 @@ class DashActor extends St.Widget {
         this.set_allocation(box, flags);
 
         let [appIcons, showAppsButton] = this.get_children();
-        let [showAppsMinHeight, showAppsNatHeight] = showAppsButton.get_preferred_height(availWidth);
+        let [, showAppsNatHeight] = showAppsButton.get_preferred_height(availWidth);
 
         let childBox = new Clutter.ActorBox();
         childBox.x1 = contentBox.x1;