Remove separate result set for Type requests.

Because results are now filtered by the tracker:available flag the
sparql query path is always used for type queries and
QGalleryTrackerResultSet is sufficient for those so take the opportunity
to shed some unneeded code.

Change-Id: Ib44dbcffeeaa2be8015ca6a727a45e19234f5782
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>