Remove qtdemo from gitignore

QtDemo was removed with a commit in qtdoc.

Change-Id: I6e0f6297dd39becd516bcbaf94bd1c1fc47f5767
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>
Merge remote-tracking branch 'origin/5.4' into dev

Change-Id: I0520ca0382bd378650600d6985a7a6c502d476c6