Following T245860 and T245865 the "repository definitions" concept and the related `RepositoryDefinitions` class are no longer used in Wikibase.
[] Remove repository definitions from `DataAccessSnakFormatterOutputFormatTest`: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/574183
[x] Remove repository definitions from `UsageTrackingIntegrationTest`: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/574185
[] Remove `WikibaseClient::getRepositoryDefinitions`:
[] https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/574186
[x] https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/ArticlePlaceholder/+/574241
[x] Remove repository definitions from `WikibaseRepo`: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/574091
[] Remove `RepositoryDefinitions` class: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/574187