code: DatanodeBlockInfo remove(int namespaceId, Block block) {
      NamespaceMap nm = getNamespaceMap(namespaceId);
      if (nm == null) {
        return null;
      }
      if (datasetDelta != null) {
        datasetDelta.removeBlock(namespaceId, block);
      }
      return nm.removeBlockInfo(block);
    }
docs: Remove the replica's meta information from the map that matches the input
  block's id and generation stamp
  
  @param namespaceId
  @param block
  block with its id as the key
  @return the removed replica's meta information
  @throws IllegalArgumentException
  if the input block is null