code: public static function removeCssClass(&$options, $class)
      {
          if (isset($options['class'])) {
              if (is_array($options['class'])) {
                  $classes = array_diff($options['class'], (array) $class);
                  if (empty($classes)) {
                      unset($options['class']);
                  } else {
                      $options['class'] = $classes;
                  }
              } else {
                  $classes = preg_split('/\s+/', $options['class'], -1, PREG_SPLIT_NO_EMPTY);
                  $classes = array_diff($classes, (array) $class);
                  if (empty($classes)) {
                      unset($options['class']);
                  } else {
                      $options['class'] = implode(' ', $classes);
                  }
              }
          }
      }
docs: Removes a CSS class from the specified options.
  @param array $options the options to be modified.
  @param string|array $class the CSS class(es) to be removed
  @see addCssClass()