docker/services: Fix swh-objstorage entrypoint
aiohttp use has been removed in favor of a flask RPC server in swh-objstorage by rDOBJS32cfda0406aa2a8b9f4727abf2ea56b7563460ea.
So we need to remove the use of the aiohttp.worker.GunicornWebWorker worker class in gunicorn configuration as the following error is now reported:
"wsgi app should be either Application or async function returning Application".
Migrated from D7307 (view on Phabricator)