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".