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/appDisplay.js

@@ -1109,7 +1109,7 @@ var FolderView = class FolderView extends BaseAppView {
     }
 
     usedWidth() {
-        let [availWidthPerPage, availHeightPerPage] = this._getPageAvailableSize();
+        let [availWidthPerPage] = this._getPageAvailableSize();
         return this._grid.usedWidth(availWidthPerPage);
     }