[Data] Remove obsolete `_DatasetWrapper`

[bveeramani]

The StreamingSplitDataIterator wraps its underlying dataset with a _DatasetWrapper class to avoid #52549. Since that issue has been now fixed, we should clean up the code and remove the unnecessary wrapper:

ray/python/ray/data/_internal/iterator/stream_split_iterator.py

Lines 28 to 33 in 631684a

	class _DatasetWrapper:
	# A temporary workaround for https://github.com/ray-project/ray/issues/52549
	def __init__(self, dataset: "Dataset") -> None:
	self._dataset = dataset