Remove dubious QMAKE_TARGET_PRODUCT, *_DESCRIPTION

This is a library, so the defaults should be fine.

Change-Id: I03a663ce62a8f2604ed01454cb5824a4d341c080
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>