Fix a bug in DatePicker when using valueLink

[leesiongchan]

This fix will remove valueLink to avoid Invariant Violation: Cannot provide a valueLink and a value or onChange event. If you want to use value or onChange, you probably don't want to use valueLink.

[hai-cea]

A better way to do this would be to add valueLink inside the spread operator. However, in looking at the code, I don't think this component is handling valueLink properly since it should return the date as the first parameter in the onChange callback. Also, it should call valueLink.requestChange. This will have to be addressed in a separate PR.

[leesiongchan]

Updated.

[hai-cea]

Thanks @ahlee2326

[ee7klt]

is this related to the error I'm getting when trying to define requestChange on datepicker? the error i'm getting is this.props.valueLink.requestChange is not a function. Just curious, why wouldn't i want to use valueLink access changes like I can do in most of the other components? I've used valueLink successfully on selectField and textField.