add firewall plugin (no backend) to default cni config

in order for the fall back mechanisms to work in containernetworking-plugins, the firewall plugin must still be called via the cni configuration file.  however, no backend w

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

cni/87-podman-bridge.conflist

@@ -31,8 +31,7 @@
             }
 	},
 	{
-            "type": "firewall",
+            "type": "firewall"
-	    "backend": "iptables"
 	}
     ]
 }