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.

pkg/rpc/server/server_helper_test.go

@@ -89,7 +89,7 @@ func getTestBlocks(t *testing.T) []*block.Block {
 	blocks := make([]*block.Block, 0, int(nBlocks))
 	for i := 0; i < int(nBlocks); i++ {
 		_ = br.ReadU32LE()
-		b := block.New(netmode.UnitTestNet, false)
+		b := block.New(false)
 		b.DecodeBinary(br)
 		require.Nil(t, br.Err)
 		blocks = append(blocks, b)