syntax for remove_filter in parent theme with class

It looks like the plugin doesn’t want you do access the APP_Registration instance, so you can try this:

add_filter( 'register_url', function( $url )
{
    // Adjust this to your needs
    $url = site_url( 'wp-login.php?action=register', 'login' );

    return $url;
}, 11 );

to override the plugin’s modifications, where we use a priority > 10.