Remove "Split summary at cursor" button

function foo_form_alter(&$form, $form_state, $form_id) {
  if ($form['#id'] == 'node-form') {
    if (!empty($form['body_field'])) {
      $form['body_field']['teaser_js']['#type'] = 'hidden';
      $form['body_field']['teaser_include']['#type'] = 'hidden';
    }
  }
}