When disabling ATProto for an account, remove its DNS record

[snarfed]

Wouldn't save us a ton of DNS space, but maybe something!

[snarfed]

Backfilled this for web, see #1458 (comment). Now backfilling for AP with:

ids = []
for a in ActivityPub.query(ActivityPub.copies.protocol == 'atproto'):
  if a.is_enabled(ATProto):
    continue
  ids.append(a.key.id())
  print(len(ids), a.handle)
  try:
    did = a.get_copy(ATProto)
    repo = arroba.server.storage.load_repo(did)
    arroba.server.storage.write_event(repo=repo, type='account',
                                      active=False, status='deactivated')
    ATProto.remove_dns(a.handle_as('atproto'))
  except:
    logging.exception('')