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/network/payload/headers.go

@@ -4,15 +4,13 @@ import (
 	"errors"
 	"fmt"
 
-	"github.com/nspcc-dev/neo-go/pkg/config/netmode"
 	"github.com/nspcc-dev/neo-go/pkg/core/block"
 	"github.com/nspcc-dev/neo-go/pkg/io"
 )
 
 // Headers payload.
 type Headers struct {
-	Hdrs    []*block.Header
-	Network netmode.Magic
+	Hdrs []*block.Header
 	// StateRootInHeader specifies whether header contains state root.
 	StateRootInHeader bool
 }
@@ -48,7 +46,6 @@ func (p *Headers) DecodeBinary(br *io.BinReader) {
 
 	for i := 0; i < int(lenHeaders); i++ {
 		header := &block.Header{}
-		header.Network = p.Network
 		header.StateRootEnabled = p.StateRootInHeader
 		header.DecodeBinary(br)
 		p.Hdrs[i] = header