Remove triple where Metadata is the object

        :param subj: Subject to match, None to match all
        :param predicate: Predicate to match, None to match all

def unlink(self, subj=None, predicate=None):
        """ Remove triple where Metadata is the object

        :param subj: Subject to match, None to match all
        :param predicate: Predicate to match, None to match all
        """
        self.graph.remove((subj, predicate, self.asNode()))