Drop RpcProxy usage from nec plugin

This patch removes the usage of the RpcProxy compatibility class from
the nec plugin.  The equivalent usage of oslo.messaging APIs is now
used instead.

Part of blueprint drop-rpc-compat.

Change-Id: I0b3ee1437ad4525f1354caaba686b6ab7442db2f
Merge "Drop RpcProxy usage from nec plugin"