Drop old code from SecurityGroupAgentRpcApiMixin

The SecurityGroupAgentRpcApiMixin class temporarily had support for
both RpcProxy and new oslo.messaging style rpc client interfaces.  Now
that all users of this mixin have been converted to direct usage of
oslo.messaging APIs, the old compatibility code can be removed.

Part of blueprint drop-rpc-compat.

Change-Id: I18dc65e7f14a8ae42e3c2bc81d1d73f61c12c1fc
Merge "Drop old code from SecurityGroupAgentRpcApiMixin"