theme_checkbox() initialize a variable that is never used

Drupal 8 doesn't define anymore theme_checkbox(). (It's Drupal API that is not updated.)

This is still true for Drupal 7, where theme_checkbox() uses the following code.

  $element = $variables['element'];
  $t = get_t();
  $element['#attributes']['type'] = 'checkbox';
  element_set_attributes($element, array('id', 'name', '#return_value' => 'value'));

  // Unchecked checkbox has #value of integer 0.
  if (!empty($element['#checked'])) {
    $element['#attributes']['checked'] = 'checked';
  }
  _form_set_class($element, array('form-checkbox'));

  return '<input' . drupal_attributes($element['#attributes']) . ' />';