Remove a child from a PID concept.

def remove_child(self, child_pid):
        """Remove a child from a PID concept."""
        with db.session.begin_nested():
            if not isinstance(child_pid, PersistentIdentifier):
                child_pid = resolve_pid(child_pid)
            relation = PIDRelation.query.filter_by(
                parent=self._resolved_pid,
                child=child_pid,
                relation_type=self.relation_type.id).one()
            db.session.delete(relation)