remove optional parameters from $flag->theme()

   * @param $action
   *  The action the link is about to carry out, either "flag" or "unflag".
   * @param $entity_id
   *  The ID of the object to flag.
   * @param $after_flagging = FALSE
   *  (Optional) Set to TRUE if this flag link is being displayed as the result
   *  of a flagging action.
   * @param $errors = array()
   *  An array of error messages.
   * @param $variables = array()
   *  An array of further variables to pass to theme('flag'). For parameters
   *  documentation, see flag.tpl.php
   *
   * @return
   *  The HTML for the flag link.
   */
  function theme($action, $entity_id, $after_flagging = FALSE, $errors = array(), $variables = array()) {

#1892930: Placement of flag links as pseudofields is about to add the $variables parameter.

This would be a good opportunity to remove the optional parameters and fold them into $variables.