remove bus-proxyd

remove bus-proxyd

As kdbus won't land in the anticipated way, the bus-proxy is not needed in
its current form. It can be resurrected at any time thanks to the history,
but for now, let's remove it from the sources. If we'll have a similar tool
in the future, it will look quite differently anyway.

Note that stdio-bridge is still available. It was restored from a version
prior to f252ff17, and refactored to make use of the current APIs.