Retire build-sunet-a.torproject.net

build-sunet-a.torproject.net has been our build machine for many years and it served us well. We have two build machines on TPO infra now (see: #34122 (closed)) and don't need nor use it anymore. I think it can be retired (unless @boklm thinks otherwise)

checklist:

1. announcement
2. nagios (not present)
3. retire the host in fabric
4. remove from LDAP with ldapvi (not in ldap)
5. power-grep (only in DNS)
6. remove from tor-passwords (nothing there)
7. remove from DNSwl (N/A)
8. remove from docs (not found in the grep, not documented in inventory (!?))
9. remove from racks (deleted in openstack)
10. remove from reverse DNS