Remove get_network from nova.network.rpcapi

Remove the get_network() method from nova.network.rpcapi.  It's not
actually used, so help make that clear by removing it from the client
rpc class.

This also helps us clarify what changes we have to be careful about with
respect to rpc versioning.

Related to nova-network-objects

Change-Id: Ibfcc90499fb6c20b7f14597ea360ed55bb34e058
Merge "Remove get_network from nova.network.rpcapi"