retire prometheus2 server

Once the prometheus1 server has scraped the prometheus2 server for long enough (see the retention period, #41637), retire prometheus2. Usual checklist:

1. announcement
2. nagios
3. retire the host in fabric
4. remove from LDAP with ldapvi
5. power-grep
6. remove from tor-passwords
7. remove from DNSwl
8. remove from docs
9. remove from racks
10. remove from reverse DNS