Update index.ios.ts to remove callback on cancel of DateTimePicker dialog.

[mrwrighty]

Removed callback(null) for cancelAction in iOS. If left in this results in datePickerClosed event being called.

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[mrwrighty]

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[NathanWalker]

Isn't a cancel a close though? Just wondering without anything to notify when canceled would leave out a way to respond to when the datepicker is no longer on screen, which you may want if doing other ui or data reactions when showing the datepicker. Idk?

[mrwrighty]

Cancel is a close but still calls onDateChanged even though callback(null) is used. By removing the callback(null) the datetimepicker just closes with no event.