Drop usage of RpcProxy from DhcpPluginApi

Drop the usage of the RpcProxy compatibility class from the
DhcpPluginApi RPC client class.  The implementation has been updated
to use the appropariate APIs from oslo.messaging directly.

Part of blueprint drop-rpc-compat.

Change-Id: I81bfd761707c4c587b12877668c4399efe3d652e