gallery: get rid of the dummy child window

This is material for a manual _desktop_ test application.

Change-Id: I9fe10ca05184410b53331bdd7b3157daaceb7640
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>