Replace containsSet() with QSet::contains()

We have a QSet::contains() since 4.6.

Change-Id: Idbafeec6e58aadff4a07e7ed50ac54dc8738623a
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>