code: func dbRemoveBlockIDIndexEntry(dbTx database.Tx, hash *chainhash.Hash) error {
  	// Remove the block hash to ID mapping.
  	meta := dbTx.Metadata()
  	hashIndex := meta.Bucket(idByHashIndexBucketName)
  	serializedID := hashIndex.Get(hash[:])
  	if serializedID == nil {
  		return nil
  	}
  	if err := hashIndex.Delete(hash[:]); err != nil {
  		return err
  	}
  
  	// Remove the block ID to hash mapping.
  	idIndex := meta.Bucket(hashByIDIndexBucketName)
  	return idIndex.Delete(serializedID)
  }
docs: // dbRemoveBlockIDIndexEntry uses an existing database transaction remove index
  // entries from the hash to id and id to hash mappings for the provided hash.