[multi-sources-removal] drop repository entities_modified_since public api actually only used by pyrorql/zmqrql sources which have been dropped. This will allow to drop a bunch of system meta-data (eg deleted_entities) Related to #2919300

Essayer le champ de recherche. Essayer des recherches comme "fiche
unmotachercher", ou encore "fiche wikiid index" ou "345".