Drop RpcProxy usage from cisco.l3

Drop usage of the RpcProxy compatibility class from cisco.l3.  This
was the last of RpcProxy usage in the cisco plugin.  The equivalent
APIs from oslo.messaging are now used instead.

Part of blueprint drop-rpc-compat.

Change-Id: I340b6969e32ec20cb4864021ec332eb9c5bdf19c
Merge "Drop RpcProxy usage from cisco.l3"