Mailcow: Remove any traces of IPv6.

In the Kata container pod that is used to run Mailcow, ip6tables fails
with messages such as:

    ipv6nat-mailcow_1    running [/sbin/ip6tables -t filter -S --wait]: exit status 3: ip6tables v1.8.4 (legacy): can't initialize ip6tables table `filter': Table does not exist (do you need to insmod?)
    ipv6nat-mailcow_1    Perhaps ip6tables or your kernel needs to be upgraded.

This is causing the Netfilter service to fail, breaking fail2ban.
diff --git a/mailcow/src/mailcow-dockerized/docker-compose.yml b/mailcow/src/mailcow-dockerized/docker-compose.yml
index ff1e5eb..72c8a20 100644
--- a/mailcow/src/mailcow-dockerized/docker-compose.yml
+++ b/mailcow/src/mailcow-dockerized/docker-compose.yml
@@ -382,7 +382,7 @@
             - acme
 
     netfilter-mailcow:
-      image: mailcow/netfilter:1.38
+      build: ./data/Dockerfiles/netfilter
       stop_grace_period: 30s
       depends_on:
         - dovecot-mailcow
@@ -517,36 +517,6 @@
           aliases:
             - olefy
 
-    ipv6nat-mailcow:
-      depends_on:
-        - unbound-mailcow
-        - mysql-mailcow
-        - redis-mailcow
-        - clamd-mailcow
-        - rspamd-mailcow
-        - php-fpm-mailcow
-        - sogo-mailcow
-        - dovecot-mailcow
-        - postfix-mailcow
-        - memcached-mailcow
-        - nginx-mailcow
-        - acme-mailcow
-        - netfilter-mailcow
-        - watchdog-mailcow
-        - dockerapi-mailcow
-        - solr-mailcow
-      environment:
-        - TZ=${TZ}
-      image: robbertkl/ipv6nat
-      security_opt:
-        - label=disable
-      restart: always
-      privileged: true
-      network_mode: "host"
-      volumes:
-        - /var/run/docker.sock:/var/run/docker.sock:ro
-        - /lib/modules:/lib/modules:ro
-
 networks:
   mailcow-network:
     driver: bridge