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/panelMenu.js

@@ -64,7 +64,7 @@ class ButtonBox extends St.Widget {
         if (!child)
             return;
 
-        let [minWidth, natWidth] = child.get_preferred_width(-1);
+        let [, natWidth] = child.get_preferred_width(-1);
 
         let availWidth = box.x2 - box.x1;
         let availHeight = box.y2 - box.y1;