tracker: do not rely use nie:isStoredAs as a property reflecting URN
tracker: do not rely use nie:isStoredAs as a property reflecting URN

This is obviously a wrong usage of nie:isStoredAs since it's not
suppose to follow the below relation :

            ?urn = nie:isStoredAs(?urn)

Signed-off-by: Lionel Landwerlin <9463032a5b99ffad10f36631678a833369a311b5@linux.intel.com>

GNOME/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,vrutkovs/grilo-plugins,jasuarez/grilo-plugins,grilofw/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,kyoushuu/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,GNOME/grilo-plugins,grilofw/grilo-plugins,jasuarez/grilo-plugins,kyoushuu/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,kyoushuu/grilo-plugins