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