Unlink a user from an external id.

    :param external_id: The external id associated with the user.

def oauth_unlink_external_id(external_id):
    """Unlink a user from an external id.

    :param external_id: The external id associated with the user.
    """
    with db.session.begin_nested():
        UserIdentity.query.filter_by(id=external_id['id'],
                                     method=external_id['method']).delete()