core/types: remove unused field 'td' in Block (ethereum#25010)

Author: zimbabao

core/types/block.go

@@ -172,10 +172,6 @@ type Block struct {
 	hash atomic.Value
 	size atomic.Value
 
-	// Td is used by package core to store the total difficulty
-	// of the chain up to and including the block.
-	td *big.Int
-
 	// These fields are used by package eth to track
 	// inter-peer block relay.
 	ReceivedAt   time.Time
@@ -197,7 +193,7 @@ type extblock struct {
 // are ignored and set to values derived from the given txs, uncles
 // and receipts.
 func NewBlock(header *Header, txs []*Transaction, uncles []*Header, receipts []*Receipt, hasher TrieHasher) *Block {
-	b := &Block{header: CopyHeader(header), td: new(big.Int)}
+	b := &Block{header: CopyHeader(header)}
 
 	// TODO: panic if len(txs) != len(receipts)
 	if len(txs) == 0 {