docs: Remove firewalld zone instructions

These were related to migrating to the `docker` zone support in new moby releases since integration landed in 2020H2.

network/iptables.md

@@ -100,13 +100,3 @@ If you are running Docker with [firewalld](https://firewalld.org){: target="blan
 on your system with `--iptables` enabled, Docker automatically creates a `firewalld`
 zone called `docker` and inserts all the network interfaces it creates (for example,
 `docker0`) into the `docker` zone to allow seamless networking.
-
-Consider running the following `firewalld` command to remove the docker interface from the zone.
-
-```console
-# Please substitute the appropriate zone and docker interface
-$ firewall-cmd --zone=trusted --remove-interface=docker0 --permanent
-$ firewall-cmd --reload
-```
-
-Restarting `dockerd` daemon inserts the interface into the `docker` zone.