[multi-sources-removal] Drop deleted_entities system table and entities.mtime column since they were only used by the entities_modified_since api of the repository which has been dropped. Along with them, the multi-sources-etypes configuration variable and some sql queries at modification/deletion time of entities. Bon vent ! Related to #2919300

raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")