Simplify `file_utils.list_dataset_variants()`. #10495

Job	Run time
Check whether we should run tests or not	10s
HuggingFace Python 3.10 tests	2m 3s
Github API tests	2m 20s
Notebook tests	6m 55s
Core TFDS tests / Generate shards	1s
Core TFDS tests / [Python 3.10][1/4]	5m 1s
Core TFDS tests / [Python 3.11][1/4]	4m 49s
Core TFDS tests / [Python 3.12][1/4]	5m 42s
Core TFDS tests / [Python 3.10][2/4]	4m 25s
Core TFDS tests / [Python 3.11][2/4]	4m 2s
Core TFDS tests / [Python 3.12][2/4]	4m 59s
Core TFDS tests / [Python 3.10][3/4]	4m 47s
Core TFDS tests / [Python 3.11][3/4]	4m 42s
Core TFDS tests / [Python 3.12][3/4]	5m 17s
Core TFDS tests / [Python 3.10][4/4]	4m 56s
Core TFDS tests / [Python 3.11][4/4]	4m 41s
Core TFDS tests / [Python 3.12][4/4]	5m 45s
	1h 10m 35s

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