code: def down(db, dataset_name):
      match_d = {"name": dataset_name}
      dataset_dict = db.datasets.find_one(match_d)
  
      dataset_dict.pop("annotation_runs", None)
  
      brain_methods = dataset_dict.get("brain_methods", {})
      for run_doc in brain_methods.values():
          config = run_doc.get("config", {})
          config_cls = config.get("cls", "")
          if config_cls == "fiftyone.brain.similarity.SimilarityConfig":
              config.pop("model", None)
              config.pop("metric", None)
  
      db.datasets.replace_one(match_d, dataset_dict)
  
label_desc: Use image node_id 2 for calling a global function with example usage: down(db, dataset_name) without return types