src: remove deps on ip[6]tables/firewalld.service from systemd units

src: remove deps on ip[6]tables/firewalld.service from systemd units

The unit files both have After=network.target, and this in turn implies
After=network-pre.target. Both iptables.service & ip6tables.service have
Before=network-pre.target since Fedora >= 35 and RHEL >= 8.4.

When we first added the deps on ip[6]tables.service in

  commit 0756415f147dda15a417bd79eef9a62027d176e6
  Author: Laine Stump <laine@redhat.com>
  Date:   Fri May 1 00:05:50 2020 -0400

    systemd: start libvirtd after firewalld/iptables services

the Before=network-pre.target didn't exist, but we can rely on it now
given our supported platforms matrix.

The firewalld.service has similarly has a Before=network-pre.target,
even when we took that commit above, so this dep was in face never
actually needed. This answers the question posed in that above commit
message about firewalld ordering.

https://gitlab.com/libvirt/libvirt/-/issues/489
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>