Simplify `file_utils.list_dataset_variants()`. #10500

Job	Run time
Check whether we should run tests or not	11s
Github API tests	2m 7s
Notebook tests	6m 51s
HuggingFace Python 3.10 tests	2m 18s
Core TFDS tests / Generate shards	1s
Core TFDS tests / [Python 3.10][1/4]	4m 58s
Core TFDS tests / [Python 3.11][1/4]	5m 3s
Core TFDS tests / [Python 3.12][1/4]	5m 42s
Core TFDS tests / [Python 3.10][2/4]	4m 39s
Core TFDS tests / [Python 3.11][2/4]	4m 29s
Core TFDS tests / [Python 3.12][2/4]	5m 7s
Core TFDS tests / [Python 3.10][3/4]	4m 48s
Core TFDS tests / [Python 3.11][3/4]	4m 47s
Core TFDS tests / [Python 3.12][3/4]	6m 10s
Core TFDS tests / [Python 3.10][4/4]	4m 58s
Core TFDS tests / [Python 3.11][4/4]	4m 50s
Core TFDS tests / [Python 3.12][4/4]	5m 24s
	1h 12m 23s

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