altTab: Don't fade out thumbnails on destroy

The thumbnails actor `this._thumbnails` has already been destroyed when
calling `_destroyThumbnails()` from the `destroy` signal handler because
it is a child actor of the AppSwitcherPopup. So stop destroying the
thumbnails separately (fading them out inside a destroy handler wouldn't
make sense anyway).

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/167

js/ui/altTab.js

@@ -280,12 +280,10 @@ class AppSwitcherPopup extends SwitcherPopup.SwitcherPopup {
     }
 
     _onDestroy() {
-        super._onDestroy();
-
-        if (this._thumbnails)
-            this._destroyThumbnails();
         if (this._thumbnailTimeoutId != 0)
             GLib.source_remove(this._thumbnailTimeoutId);
+
+        super._onDestroy();
     }
 
     /**