code: def remove_old_files(algo_name, today, rel_path, environ=None):
      """
      remove old files from a directory
      to avoid overloading the disk
  
      Parameters
      ----------
      algo_name: str
      today: Timestamp
      rel_path: str
      environ:
  
      Returns
      -------
      error: str
  
      """
  
      error = None
      algo_folder = get_algo_folder(algo_name, environ)
      folder = os.path.join(algo_folder, rel_path)
      ensure_directory(folder)
  
      # run on all files in the folder
      for f in os.listdir(folder):
          try:
              file_path = os.path.join(folder, f)
              creation_unix = os.path.getctime(file_path)
              creation_time = pd.to_datetime(
                  creation_unix, unit="s", utc=True
              )
  
              # if the file is older than 30 days erase it
              if today - pd.DateOffset(30) > creation_time:
                  os.unlink(file_path)
          except OSError:
              error = "unable to erase files in {}".format(folder)
  
      return error
  
imports: import hashlib
  import json
  import os
  import pickle
  import shutil
  from datetime import date, datetime
  import pandas
  from catalyst.assets._assets import TradingPair
  from six import string_types
  from six.moves.urllib import request
  from catalyst.constants import DATE_FORMAT, SYMBOLS_URL
  from catalyst.exchange.exchange_errors import ExchangeSymbolsNotFound
  from catalyst.exchange.utils.serialization_utils import ExchangeJSONEncoder, ExchangeJSONDecoder
  from catalyst.utils.memoize import weak_lru_cache
  from catalyst.utils.paths import data_root, ensure_directory, last_modified_time
label_desc: Use image node_id 19 for calling a global function with example usage: remove_old_files(algo_name, today, rel_path, environ) and returns: error