Before this commit, this would make swh_storage_backend_config fixture clients
require to declare an optional dependency on swh.journal. Otherwise, it would not
This commit fixes it by dropping this configuration in the main pytest plugin.
It keeps the storage tests testing with that journal_writer collaborator though (as initially)
by declaring an override which still provides it.
This fixes the debian build 
This replaces D3514 by fixing the problem at the source. D3514 took the path to
change the configuration within the clients. This is not sustainable as this
would require other fixture clients to do the same.