[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]

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)