Disabling 'Prieview' for Drupal 5.3

Hello! This is my first post and I am a beginner. Thank you for such a great forum. After searching for a long time, I found that the current methods for removing the preview function altogether by creating a new module do not work on Drupal 5.3. Does anyone know of a way to disable the preview with 5.3 or am I possibly doing something wrong?

I am creating a module with the following:

function killpreview_form_alter($form_id, &$form) {
  if ($form_id == 'killpreview_node_form') {
    unset($form['preview']);
  }
}

but it isn't removing the preview button. Thanks for your help.