Drop RpcProxy usage from L3AgentNotifyAPI

Remove RpcProxy usage from the L3AgentNotifyAPI.  The equivalent
oslo.messaging APIs are now used instead.

Part of blueprint drop-rpc-compat.

Change-Id: Ice8e2592d7b254134bbee215b93ca77075ab4fe8
Merge "Drop RpcProxy usage from L3AgentNotifyAPI"