Remove 'fivestar_select' theme function

Problem/Motivation

The theme functions are deprecated in Drupal 9 and will be removed in Drupal 10.
We don't use that function in the last version of the module and for me not clear if it really needed.
The function was added in Drupal 5 and with minimal changes was migrated to D6 and D7.

Function code from D7:

/**
 * Theme the straight HTML version of the fivestar select list. This is used
 * to remove the wrapping 'form-item' div from the select list.
 */
function theme_fivestar_select($variables) {
  $element = $variables['element'];
  element_set_attributes($element, array('id', 'name', 'size'));
  _form_set_class($element, array('form-select'));
  return '<select' . drupal_attributes($element['#attributes']) . '>' . form_select_options($element) . '</select>';
}

Proposed resolution

Remove it now to omit deprecation errors and reintroduce it in the future if someone can explain it's purpose.