Avoid multiple default config names

[albertvillanova]

Fix duplicating default config names.

Currently, when calling push_to_hub(set_default=True with 2 different config names, both are set as default.

Moreover, this will generate an error next time we try to push another default config name, raised by MetadataConfigs.get_default_config_name:

datasets/src/datasets/arrow_dataset.py

Line 5757 in da1db8a

default_config_name = metadata_configs.get_default_config_name()

datasets/src/datasets/utils/metadata.py

Lines 186 to 188 in da1db8a

	raise ValueError(
	f"Dataset has several default configs: '{default_config_name}' and '{config_name}'."
	)

[HuggingFaceDocBuilderDev]

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