Delete duckdb files when a split is deleted
See https://huggingface.co/datasets/HuggingFaceH4/ultrachat_200k/tree/refs%2Fconvert%2Fparquet/default/train

The `train` split existed, but has been deleted. Now, the datasets has `train_sft` and `train_gen`. But the duckdb index file still exists for `train` in `refs/convert/parquet`. Note that the parquet files have been deleted.
Delete duckdb files when a split is deleted: See https://huggingface.co/datasets/HuggingFaceH4/ultrachat_200k/tree/refs%2Fconvert%2Fparquet/default/train

The `train` split existed, but has been deleted. Now, the datasets has `train_sft` and `train_gen`. But the duckdb index file still exists for `train` in `refs/convert/parquet`. Note that the parquet files have been deleted.