Delete firewall doc

The firewall document is out of date in many ways:

- The package is now pulled from Debian, not a modified Fedora one
- The iptables rules have been conciderably extended
- Disabling iptables as described does not work

As a result, there's very little information here worth keeping. As we have
a more architectural overview of filtering(1), drop this page.

1) https://designs.apertis.org/latest/security.html#network-filtering



Signed-off-by: Martyn Welch <martyn.welch@collabora.com>