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.

TODO

@@ -321,10 +321,6 @@ Features:
        - path escaping
   - update systemd.special(7) to mention that dbus.socket is only about the compatibility socket now
   - test bloom filter generation indexes
-  - bus-proxy: when passing messages from kdbus, make sure we properly
-    handle the case where a large number of fds is appended that we
-    cannot pass into sendmsg() of the AF_UNIX sokcet (which only accepts
-    253 messages)
   - kdbus: introduce a concept of "send-only" connections
   - kdbus: add counter for refused unicast messages that is passed out via the RECV ioctl. SImilar to the counter for dropped multicast messages we already have.