Drop RpcProxy usage from DhcpAgentNotifyAPI

Remove usage of the RpcProxy compatibility class from the
DhcpAgentNotifyAPI class.  The equivalent oslo.messaging APIs are now
used instead.

Part of blueprint drop-rpc-compat.

Change-Id: Ib658a0d67da1af3b009bc6df9a7c8ec08c04897b
Merge "Drop RpcProxy usage from DhcpAgentNotifyAPI"