Drop RpcProxy usage from cisco apic ml2 plugin

Drop usage fo the RpcProxy compatibility class from the cisco apic ml2
plugin.  The equivalent oslo.messaging APIs are now used instead.

Part of blueprint drop-rpc-compat.

Change-Id: I415b65720a09652bae70fc1c1dada9f5727c4441
Merge "Drop RpcProxy usage from cisco apic ml2 plugin"