consensus/clique, core: API cleanup (#23100)

This removes some code:

- The clique engine calculated the snapshot twice when verifying headers/blocks.

- The method GetBlockHashesFromHash in Header/Block/Lightchain was only used by tests. It
  is now removed from the API.
  
- The method GetTdByHash internally looked up the number before calling GetTd(hash, num).
  In many cases, callers already had the number, and used this method just because it has a
  shorter name. I have removed the method to make the API surface smaller.

core/headerchain.go

@@ -394,29 +394,6 @@ func (hc *HeaderChain) InsertHeaderChain(chain []*types.Header, start time.Time)
 	return res.status, err
 }
 
-// GetBlockHashesFromHash retrieves a number of block hashes starting at a given
-// hash, fetching towards the genesis block.
-func (hc *HeaderChain) GetBlockHashesFromHash(hash common.Hash, max uint64) []common.Hash {
-	// Get the origin header from which to fetch
-	header := hc.GetHeaderByHash(hash)
-	if header == nil {
-		return nil
-	}
-	// Iterate the headers until enough is collected or the genesis reached
-	chain := make([]common.Hash, 0, max)
-	for i := uint64(0); i < max; i++ {
-		next := header.ParentHash
-		if header = hc.GetHeader(next, header.Number.Uint64()-1); header == nil {
-			break
-		}
-		chain = append(chain, next)
-		if header.Number.Sign() == 0 {
-			break
-		}
-	}
-	return chain
-}
-
 // GetAncestor retrieves the Nth ancestor of a given block. It assumes that either the given block or
 // a close ancestor of it is canonical. maxNonCanonical points to a downwards counter limiting the
 // number of blocks to be individually checked before we reach the canonical chain.
@@ -472,16 +449,6 @@ func (hc *HeaderChain) GetTd(hash common.Hash, number uint64) *big.Int {
 	return td
 }
 
-// GetTdByHash retrieves a block's total difficulty in the canonical chain from the
-// database by hash, caching it if found.
-func (hc *HeaderChain) GetTdByHash(hash common.Hash) *big.Int {
-	number := hc.GetBlockNumber(hash)
-	if number == nil {
-		return nil
-	}
-	return hc.GetTd(hash, *number)
-}
-
 // GetHeader retrieves a block header from the database by hash and number,
 // caching it if found.
 func (hc *HeaderChain) GetHeader(hash common.Hash, number uint64) *types.Header {