Doc: Made "Qt Image Formats" entry in Assistant/Creator childless.

The module doesn't really have any child documentation and it used
to just list random overviews from other modules.

Change-Id: I41021e7e649a1e0cbe430d80be55e9f601b00be0
Reviewed-by: Geir Vattekar <geir.vattekar@digia.com>