Drop usage of RpcProxy from L3PluginApi

Drop the usage of the RpcProxy compatibility class from the
L3PluginApi.  The equivalent direct usage of the oslo.messaging APIs
are now being used instead.

Part of blueprint drop-rpc-compat.

Change-Id: I6639d1aa8acca8c0544020e28489e71f3d5d2955
Merge "Drop usage of RpcProxy from L3PluginApi"