Remove obsolete postfix_null role from gemini.archlinux.org

Gemini doesn't need to send emails and hasn't had that need for several
years (AFAIU). It may have been used by the integrity-check script in
the past, but that was disabled 7 years ago[1] and was just removed
completely from dbscripts[2][3] and the infra repo[4].

[1] bcd20f65 ("Disable the integrity check timer and service.")
[2] dbscripts!58
[3] f354ae35 ("dbscripts: Bump to latest commit for repos/archive split fixes")
[4] 8e22cdbe ("dbscripts: Remove obsolete integrity-check.{service,timer}")

playbooks/gemini.archlinux.org.yml

@@ -14,7 +14,6 @@
     - { role: nginx }
     - { role: archive }
     - { role: archive_web }
-    - { role: postfix_null }
     - { role: fail2ban }
     - { role: prometheus_exporters }
     - { role: promtail }