[Bug] Avoid nat when there is only a single node at endpoints in the Gateway

[YTGhost]

Why

When there is only a single node at the endpoints in the Gateway, the NAT avoidance rules are cleaned up.

// pkg/networkengine/routedriver/vxlan/vxlan.go L490~501
err = vx.iptables.NewChainIfNotExist(iptablesutil.NatTable, iptablesutil.RavenPostRoutingChain)
if err != nil {
	errList = errList.Append(fmt.Errorf("error create %s chain: %s", iptablesutil.PostRoutingChain, err))
}
err = vx.iptables.DeleteIfExists(iptablesutil.NatTable, iptablesutil.PostRoutingChain, "-m", "comment", "--comment", "raven traffic should skip NAT", "-o", "raven0", "-j", iptablesutil.RavenPostRoutingChain)
if err != nil {
	errList = errList.Append(fmt.Errorf("error deleting %s chain rule: %s", iptablesutil.PostRoutingChain, err))
}
err = vx.iptables.ClearAndDeleteChain(iptablesutil.NatTable, iptablesutil.RavenPostRoutingChain)
if err != nil {
	errList = errList.Append(fmt.Errorf("error deleting %s chain %s", iptablesutil.RavenPostRoutingChain, err))
}