[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]
[multi-sources-removal] Drop deleted_entities system table and entities.mtime column
since they were only used by the entities_modified_since api of the
repository which has been dropped. Along with them, the multi-sources-etypes
configuration variable and some sql queries at modification/deletion time of
entities. Bon vent !
Related to #2919300