Remove no longer needed JS API data clear

[mvorisek]

introduced in #909

probably related/fixed with fomantic/Fomantic-UI#2581

I have verified the #902 is NOT reintroduced, test code:

$form = \Atk4\Ui\Form::addTo($app);
$form->addControl('purge_missing', [\Atk4\Ui\Form\Control\Checkbox::class]);
$form->buttonSave->set('Import');

$form->onSubmit(function(\Atk4\Ui\Form $form) {
    throw new \Exception((string) $form->model->get('purge_missing'));
});

(and tested by probably some Behat test already)