Have a closer look at the usage of `features` for FileDatasets

[PGijsbers]

Putting TODOs on the issue tracker instead of in code (#172).

automlbenchmark/amlb/datasets/file.py

Lines 219 to 222 in 98bf554

	features: list[ns | str]
	#TODO: DEADCODE?
	I don't see this accessed anywhere, and `features` property is retrieved
	from split metadata, which also do not reference this.

The set self._features is currently not used in the code. Could it be used for the features property instead to avoid loading the metadata? Or is no useful information passed and should it be removed from the code base because it is dead code?