Remove extra label column

[severo]

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.