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>
Merge remote-tracking branch 'origin/release' into stable

Change-Id: Ie3ca435442bcaab4d77fee973b82967741055628