Remove rpc_backend setting on newton and ocata.

This setting was deprecated for removal in newton, replaced by
transport_url.  These configurations are also setting transport_url, so
the option is ignored.

This removes a warning issued on service startup:

```
WARNING oslo_config.cfg [-] Option "rpc_backend" from group "DEFAULT" is deprecated for removal.  Its value may be silently ignored in the future.
```
4 files changed