[multi-sources-removal] Drop entities.source column
Since we remove federated multi-sources support, the same value ('system')
is always stored in this column and so could be removed. Also, cleanup
repository caches and a few api where the very same useless information could
be removed.
Closes #2919300, at last
[jcr: restore sanity check of etype against schema in checkintegrity.has_eid]
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