Drop RpcProxy usage from PluginReportStateAPI

Drop the usage of the RpcProxy compatibility class from the
PluginReportStateAPI rpc client class.  It now uses the appropriate
oslo.messaging APIs directly, instead.

Part of blueprint drop-rpc-compat.

Change-Id: I7101331a556bd0a5c8f782ae6cb4103151e6c45c
Merge "Drop RpcProxy usage from PluginReportStateAPI"