Improve metadata for `SkipCheckIfFeeless` (#7813)

If the inner transaction extension used inside `SkipCheckIfFeeless` are
multiples then the metadata is not correct, it is now fixed.

E.g. if the transaction extension is `SkipCheckIfFeeless::<Runtime,
(Payment1, Payment2)>` then the metadata was wrong.

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Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Bastian Köcher <git@kchr.de>