Plain image uploading – no links, no classes, no sizes

There’s a good answer to this question here:

His second example is pretty much what you’re looking for. Let me know if you need any further help with this.

The main chunk from that answer that concerns you is this:

function image_tag($html, $id, $alt, $title) {
    return preg_replace(array(
            'alt="' . $title . '"'
add_filter('get_image_tag', 'image_tag', 0, 4);

In your case, you’ll want the alt="' . $title . '" bit to just be '' like the strings above it.