fix(py): fix dataset created_at (#1485)

stop sending timezone-less created_at time. backend handles setting this

python/langsmith/client.py

@@ -3210,7 +3210,6 @@ def create_dataset(
         dataset: Dict[str, Any] = {
             "name": dataset_name,
             "data_type": data_type.value,
-            "created_at": datetime.datetime.now().isoformat(),
             "transformations": transformations,
             "extra": {"metadata": metadata} if metadata else None,
         }