retire legacy onionoo infrastructure

we rebuilt the onionoo service in a single machine in #41512 (closed), retire the old cluster of 4 machines.

checklist:

1. announcement
2. retire the host in fabric
3. remove from LDAP with ldapvi
4. power-grep
5. remove from tor-passwords
6. remove from DNSwl
7. remove from docs
8. remove from racks (not a physical machine)
9. remove from reverse DNS

onionoo-frontend-01, -02, onionoo-backend-01, and -02, and NOT onionoo-backend-03, which remains.