DOC: `mode` parameter on pdf and video features unused
They seem to be artefacts from a copy-paste of the Image feature ^^' we should remove them
Following up on https://github.com/huggingface/datasets/pull/7840 I asked claude code to check for undocumented parameters for other features and it found:
- mode parameter on video is documented but unused: https://github.com/huggingface/datasets/blob/main/src/datasets/features/video.py#L48-L49
- the same goes for the mode parameter on the pdf feature: https://github.com/huggingface/datasets/blob/main/src/datasets/features/pdf.py#L47-L48

I assume checking if these modes can be supported and otherwise removing them is the way to go here.
