[multi-sources-removal] Drop (dont_)cross relation related code
That was only needed for now deleted 'true' multisources.
Related to #2919300
[jcr: also remove it from server/test/data-schemaserial/schema.py]
# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s repository side views, usually for notification"""