Revert "[S32::Temporal] Expand valid ISO 8601 formats."

ISO 8601 is a good thing, but that does not mean *all* of it is
a good thing. Specifiacally, allowing week specifications in the
DateTime core will make the logic even more complicated than it
already is, with extremely little benefit.

This reverts commit 97c5bc4e47b819b640fc5f815b861d753ed8aa9c.