fix(date-picker): remove border radius

[emyarod]

Closes #8190

This PR removes the border radius of the flatpickr calendar to match the component spec

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[shixiedesign]

Hmm... I think I still see it in the preview build?

[tw15egan]

Hmm yeah, it seems like it's not picking up the border-radius from flatpickr-calendar and needs to be added to .flatpickr-calendar.open for some reason.

cc @emyarod

[emyarod]

will take another look

[emyarod]

I don't think it's related to the open class, it's just the rule a few lines down