Don't store feeding service instance in registry.

[ride90]

There is no sense to store feeding service instance in registered_feeding_services https://github.com/superdesk/superdesk-core/blob/master/superdesk/io/registry.py#L42, would be enough to store just class.
P.S. We create a new feeding service instance all the time during update_provider task https://github.com/superdesk/superdesk-core/blob/master/superdesk/io/commands/update_ingest.py#L234 so there is no sense to keep instance in the registry.