Remove DatasetWorker.generator

Depends #302 (closed)

This attribute was introduced to support workers that would actually update dataset states. In the end, this behaviour is very specific, so the code should instead live over there.

Remove any behaviour related to it in https://gitlab.teklia.com/workers/base-worker/-/blob/112551b9a853a5ae31117f46c1271d4e7617a31a/arkindex_worker/worker/init.py#L321