Disable monitoring luna.archlinux.org:25

The monitoring on luna fails randomly and we have no proof that the mail
server does not work. Disable it for now until it works in a stable
fashion.

roles/prometheus/defaults/main.yml

@@ -47,6 +47,5 @@ blackbox_targets:
     - mail.archlinux.org:993
     - mail.archlinux.org:995
   smtp_starttls:
-    - luna.archlinux.org:25
     - mail.archlinux.org:25
     - mail.archlinux.org:587