Revert "Handle Unix domain socket connections from outside our namespace"

This reverts commit 9754d56e9b21bfe89fc18f47987d6bef491b8521.

It causes a crash in PID1:
  Apr 19 13:49:32 lon systemd[1]: Code should not be reached 'Unhandled socket type.'
                                  at src/core/socket.c:684, function instance_from_socket(). Aborting.
  Apr 19 13:49:32 lon systemd[1]: Caught <ABRT>, dumped core as pid 336.
  Apr 19 13:49:32 lon systemd[1]: Freezing execution.