How to remove parent section from attachment URL?

The attachment URL is created in wp-includes/link-template.php in function get_attachment_link(). And there is a filter for you, passing the WordPress URL and the post id for the attachment. You can hook into that filter and return the URL you need:

add_filter( 'attachment_link', 'wpse_56619_unprettify_attachment_url', 10, 2 );

function wpse_56619_unprettify_attachment_url( $link, $id )
{
    return home_url( "/?attachment_id=$id" );
}