Remove get_all_networks from nova.network.rpcapi

Remove the get_all_networks() 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: I21747958bff6330b100a9fd66d855a72aeabfab8
Merge "Remove get_all_networks from nova.network.rpcapi"