Remove NetworkID from cluster

[adambabik]

Our clusters do not depend on NetworkID anymore so we can remove it to avoid duplications of enodes.

In the near future, we will add LES support and LES servers do depend on NetworkID. In such a case, it's still gonna be better to define them like this:

type Cluster struct {
	StaticNodes     []string `json:"staticnodes"`
	BootNodes       []string `json:"bootnodes"`
	MailServers     []string `json:"mailservers"` // list of trusted mail servers
	RendezvousNodes []string `json:"rendezvousnodes"`
        LESServers []struct{
                NetworkID int
                Nodes []string
        } `json:"lesservers"`
}

[jakubgs]

We eventually should start using https://fleets.status.im/ during build to fill in the data, but this is an improvement non-the-less.