[technical debt] Refactor and drop dead code from indexer

Now that the indexer are running as journal clients, new methods got opened for them in the existing scaffolding. The old ones, used from the celery task part is no longer used. This obfuscates quite a lot the code when trying to get inside it. Plus, iirc, it's not well tested as the testing started with lots of bump in there.

I should have a look and drop this dead code to simplify the maintenance.

• !488 (merged): Drop partition indexers (which are no longer deployed)
• !501 (merged): Remove swh.workers entrypoint
• Rewrite unit tests on indexer part which is currently deployed
• Drop the run method which should no longer be used.
• ...

@vlorentz might be interested to know this ;)