Don't delete QLibrarySettings configuration in ~QCoreApplication

Do not clear the QLibrarySettings configuration information already
in ~QCoreApplication (via qAddPostRoutine). This fixes issues where
multiple QCoreApplication objects are created over time (in plugins).

Task-number: QTBUG-34290
Change-Id: Ib5c58f825619ede484492e057e08d73b2b4c6101
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>