[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
Latest changes
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* table of `all latest changes`_
Links below is providing useful RQL query example.
.. _all latest changes: view?rql=Any+M%2CX+WHERE+X+modification_date+M+ORDERBY+M+DESC+LIMIT+30