network create should use firewall plugin

when creating a network, podman should add the firewall plugin to the config but not specify a backend.  this will allow cni to determine whether it should use an iptables|firewalld backend.

Signed-off-by: Brent Baude <bbaude@redhat.com>

pkg/network/netconflist.go

@@ -110,7 +110,6 @@ func NewPortMapPlugin() PortMapConfig {
 func NewFirewallPlugin() FirewallConfig {
 	return FirewallConfig{
 		PluginType: "firewall",
-		Backend:    "iptables",
 	}
 }