'samba-tool domain demote --remove-other-dead-server' fails to remove an old Windows AD DC:

  File "bin/python/samba/remove_dc.py", line 445, in remove_dc
    remove_dns_account=True)
  File "bin/python/samba/remove_dc.py", line 368, in offline_remove_ntds_dc
    remove_dns_account=remove_dns_account)
  File "bin/python/samba/remove_dc.py", line 245, in offline_remove_server
    samdb.delete(server_dn)
LdbError: (66, 'subtree_delete: Unable to delete a non-leaf node (it has 1 children)!')
A transaction is still active in ldb context [0x22f4690] on /home/timbeale/code/samba/backup_temp_dir/private/sam.ldb
ERROR(ldb): uncaught exception - subtree_delete: Unable to delete a non-leaf node (it has 1 children)!
  File "bin/python/samba/netcmd/__init__.py", line 177, in _run
    return self.run(*args, **kwargs)
  File "bin/python/samba/netcmd/domain_backup.py", line 55, in inner
    raise e

The problem happens trying to delete the old server object, i.e.
CN=<WINDOWS-DC>,CN=Servers,CN=Default-First-Site-Name,CN=Sites,CN=Configuration,<DOMAIN>

The problem is that Windows has an extra 'DNS Settings' child object underneath the server object, whereas samba doesn't. i.e.
CN=DNS Settings,CN=<WINDOWS-DC>,CN=Servers,CN=Default-First-Site-Name,CN=Sites,CN=Configuration,<DOMAIN>

Reported on mailing list here:
https://lists.samba.org/archive/samba/2018-June/216572.html

Problem also found while testing the backup/restore tool.