Disable our usage of the koji proxyuser feature.
    
    Unfortunately, the proxyuser method requires that we add all of the
    modularity developers to the ProxyDNs entry in the staging koji-hub
    config, and that's just not going to happen.
    
    We get to re-work this in this ticket anyways,
    https://pagure.io/fm-orchestrator/issue/112
    
    While the proxyuser argument is in place, we can submit no builds to
    koji at all (as far as I can tell), so this blocks further development.