function mytheme_form_user_login_form_alter(&$form, DrupalCoreFormFormStateInterface $form_state, $form_id) {

  if (isset($form['name']) && $form['name']['#type'] == 'textfield') {
    unset($form['name']['#title'], $form['name']['#description']);
  }

  if (isset($form['pass']) && $form['name']['#type'] == 'textfield') {
    unset($form['pass']['#title'], $form['pass']['#description']);
  }

}