[multi-sources-removal] drop source location search on glob_*[entity|relation] methods
source is now always the system source
Related to #2919300
[jcr: fix wrong rebase in symmetric relation removal]
[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]