[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]
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile