Remove unused property in response object.

I made sure that only one of the `result` or `error` will exist.
via. http://www.jsonrpc.org/specification#response_object
Oh really? I'll admit I only did a cursory search, but is there a good integration test suite for JSON-RPC 2.0 that we could run multitransport-jsonrpc through? (If not, I think I'll start working on such a project.)

And thank you for getting this past the last mile on 2.0 compatibility. :) It's unfortunately not been a very high priority internally, since the current code has worked just fine.