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>

group_vars/all/main.yml

@@ -2668,22 +2668,14 @@ git_default_repositories:
 # ---
 # group [oopen_server]
 # ---
-git_oopen_server_repositories:
-
-  # firewall
-  - name: ipt-server
-    repo: https://git.oopen.de/firewall/ipt-server
-    dest: /usr/local/src/ipt-server
+# (ipt-server firewall is installed by the role 'ipt-server')
+git_oopen_server_repositories: []
 
 # ---
 # group [warenform_server]
 # ---
-git_warenform_server_repositories:
-
-  # firewall
-  - name: ipt-server
-    repo: https://git.oopen.de/firewall/ipt-server
-    dest: /usr/local/src/ipt-server
+# (ipt-server firewall is installed by the role 'ipt-server')
+git_warenform_server_repositories: []
 
 # ---
 # group [lxc_host]