Remove extra `label` column

In example dataset https://huggingface.co/datasets/datasets-examples/doc-audio-4, we have an "unexpected" label column with only `null` values.

I think it's due to a "collision" between the heuristics that define splits and/or classes based on the directories. There is a `drop_labels=True` option in the datasets library, if it helps.

Ideally, in this case, we should have two splits (train and test), and no additional `label` column.

I think the issue also exists with image datasets.

Reproduce the behavior with the doc-audio-4 dataset and inspect the dataset-viewer path that infers splits and class labels from directories. Check how the datasets library's drop_labels=True option relates to this flow, then verify that the example exposes only train and test splits with no extra label column, including for image datasets.

Written by the indexing model from the issue text.