Once T214557 is finished (functionality is written and complete) and useEntitySourceBasedFederation is true everywhere in production and by default in Wikibase we can remove the old code that it is switching away from.
[x] {T245547}
[x] Remove all conditional code paths in Repo and Client that were differing depending on the value of `useEntitySourceBasedFederation` setting
[x] {T245817}
[x] {T245830}
[x] {T245831}
[x] {T245860}
[x] Remove `ForeignEntityValidator`: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/574018
[x] {T245865}
[x] Remove `UnusableEntitySource`: https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/Wikibase/+/574192
[x] {T245870}
[] Repo: Rename `WikibaseRepo::getEntityTypeToRepositoryMapping` - given "repository" concept will be gone
[] Remove the logic setting up "entity source" config from "legacy" setting options (`repositories`, `foreignRepositories`, `conceptBaseUri`, `entityNamespaces`, ...)
[x] Update services documentation https://gerrit.wikimedia.org/r/c/577313
[] (possibly optional) Remove `EntityId::getRepositoryName` and "repository prefix" related code in Wikibase Data Model library