block: drop Network from the Header

It's not network-tied any more, network is only needed to
sign/verify. Unfortunately we still have to keep network in consensus data
structures because of dbft library interface.

scripts/gendump/main.go

@@ -161,7 +161,6 @@ func newBlock(bc *core.Blockchain, lastBlock *block.Block, script []byte, txs ..
 	witness := transaction.Witness{VerificationScript: script}
 	b := &block.Block{
 		Header: block.Header{
-			Network:       netmode.UnitTestNet,
 			PrevHash:      lastBlock.Hash(),
 			Timestamp:     uint64(time.Now().UTC().Unix())*1000 + uint64(lastBlock.Index),
 			Index:         lastBlock.Index + 1,