Retire old server firewall management (roles firewall/ipt-server part, modify-ipt-server).

All ipt servers are now managed by the role ipt-server (playbook ipt-server.yml):
- roles/firewall: only handles gateways now.
- Remove playbook modify-ipt-server.yml and role modify-ipt-server.
- Remove git_firewall_repository (ipt-server) and the ipt-server entries of
  git_{oopen,warenform}_server_repositories, so role common no longer
  clones /usr/local/src/ipt-server. Gateways keep ipt-gateway.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

roles/firewall/tasks/main.yml

@@ -3,14 +3,8 @@
 # tags supportetd inside caching-nameserver.yml
 #
 # apt-caching-nameserver
-- import_tasks: ipt-server.yml
-  when: 
-     - inventory_hostname not in groups['gateway_server']
-     - inventory_hostname not in groups['no_ipt_firewall']
-  tags:
-     - git-firewall-repository
-     - ipt-server
-     
+# Servers (ipt-server) are managed by the role 'ipt-server' (playbook
+# ipt-server.yml); this role only handles gateways.
 
 - import_tasks: ipt-gateway.yml
   when: