3.5.1 Compatibility

I tried the plugin as well, and it has no effect at all, here is how I rewrite it :

add_action('after_setup_theme', 'sushi_after_setup_theme');

function sushi_after_setup_theme()
{
    remove_shortcode('gallery');
    add_shortcode('gallery', 'sushi_link_none_gallery_shortcode', 10, 1);
}

function sushi_link_none_gallery_shortcode($attr)
{
    $output = gallery_shortcode($attr);

    if($attr['link'] == "none") {
        $output = preg_replace(array('/<a[^>]*>/', '/<\/a>/'), '', $output);
    }

    return $output;
}

The important part in this code is to place your code in an after_setup_theme handler.