TPA-RFC-23: retire ipv6only.torproject.net

that server is undocumented and will eventually block Sunet's VM migration, see also #40684 (closed)

checklist:

1. announcement
2. nagios (not present)
3. retire the host in fabric
4. remove from LDAP with ldapvi (not present)
5. power-grep (not present)
6. remove from tor-passwords (not present)
7. remove from DNSwl (not present)
8. remove from docs (not present)
9. remove from racks
10. remove from reverse DNS (N/A?)