Remove returnFormat prop from DatePicker

[veej]

@gabro is proposing to remove returnFormat (and Ts other than Date) completely

Originally posted by @giogonzo in https://github.com/buildo/react-components/pull/1333

[gabro]

My proposal is a bit more radical:

• remove parametricity from DatePicker and use Date as input (value, defaultValue) and output (onChange). We can keep using moment internally, since react-dates uses it

• remove returnFormat: it seems a mix of concerns, the DatePicker should consume and yield values, whereas the presentation of those values should be handled separately

• remove onClear and just return onChange(value: Date | null) I'm not sure of what's the value of having onClear and it complicates the logic (especially in the date range picker I'm working on)