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.

eth/api_backend.go

@@ -194,7 +194,10 @@ func (b *EthAPIBackend) GetLogs(ctx context.Context, hash common.Hash) ([][]*typ
 }
 
 func (b *EthAPIBackend) GetTd(ctx context.Context, hash common.Hash) *big.Int {
-	return b.eth.blockchain.GetTdByHash(hash)
+	if header := b.eth.blockchain.GetHeaderByHash(hash); header != nil {
+		return b.eth.blockchain.GetTd(hash, header.Number.Uint64())
+	}
+	return nil
 }
 
 func (b *EthAPIBackend) GetEVM(ctx context.Context, msg core.Message, state *state.StateDB, header *types.Header, vmConfig *vm.Config) (*vm.EVM, func() error, error) {