In class-piklist-form.php, we enqueue the style:
wp_enqueue_style('jquery-ui-core');
to remove, add this to your functions.php file:
wp_dequeue_style('jquery-ui-core');

If you want to remove it on the frontend only, something like this should work (untested):
function my_scripts()
{
wp_dequeue_style('jquery-ui-core');
}
add_action('wp_enqueue_scripts','my_scripts',99);