Remove code to update Network.enableIPv6 if false

Commit 4b9dc647 (from 2016) added this in response to a
review comment, but I don't think it can ever have worked
because n.enableIPv6 = netMap["enableIPv6"].(bool), added
in 2015, would already have panic'd if the field wasn't
stored.

In any case, it's no longer needed.

Signed-off-by: Rob Murray <rob.murray@docker.com>

libnetwork/network.go

@@ -698,10 +698,6 @@ func (n *Network) UnmarshalJSON(b []byte) (err error) {
 	if v, ok := netMap["loadBalancerMode"]; ok {
 		n.loadBalancerMode = v.(string)
 	}
-	// Reconcile old networks with the recently added `--ipv6` flag
-	if !n.enableIPv6 {
-		n.enableIPv6 = len(n.ipamV6Info) > 0
-	}
 	return nil
 }