Remove neutron.common.rpc.RemoteError

Remove RemoteError from neutron.common.rpc, which was just mapped
directly to oslo.messaging.RemoteError.

Part of blueprint drop-rpc-compat.

Change-Id: Ic4d7ea0adfd68eb56100e32ce0d8e59aa2aae5b1
Merge "Remove neutron.common.rpc.RemoteError"