code: void deleteCheckpoint(long txId) throws IOException {
      for (Iterator<StorageDirectory> it = storage.dirIterator(); it.hasNext();) {
        StorageDirectory sd = it.next();
        // image file
        File imageFile = NNStorage.getImageFile(sd, txId);
        if (imageFile.delete())
          LOG.info("Delete checkpoint: deleted: " + imageFile);
        
        // md5 file
        File imageFileMD5 = MD5FileUtils.getDigestFileForFile(imageFile);
        if (imageFileMD5.delete())
          LOG.info("Delete checkpoint: deleted: " + imageFileMD5);
        
        // image ckpt file
        File imageCkptFile = NNStorage.getCheckpointImageFile(sd, txId);
        if (imageCkptFile.delete())
          LOG.info("Delete checkpoint: deleted: " + imageCkptFile);
      }
    }
docs: Deletes the checkpoint file in every storage directory,
  since the checkpoint was cancelled. Attepmts to remove
  image/md5/ckptimage files.