Drop RpcProxy usage from MeteringAgentNotifyAPI

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

Part of blueprint drop-rpc-compat.

Change-Id: I6c1fddb6694f74021b9f6a54885657916bda2875
Merge "Drop RpcProxy usage from MeteringAgentNotifyAPI"