Remove orphaned line of code from menu_get_active_help().

There's a variable $empty_arg declared in menu_get_active_help() that is not used. Here's the whole function, since it's short:

/**
 * Returns the help associated with the active menu item.
 */
function menu_get_active_help() {
  $output = '';
  $router_path = menu_tab_root_path();
  // We will always have a path unless we are on a 403 or 404.
  if (!$router_path) {
    return '';
  }

  $arg = drupal_help_arg(arg(NULL));
  $empty_arg = drupal_help_arg();

  foreach (module_implements('help') as $module) {
    $function = $module . '_help';
    // Lookup help for this path.
    if ($help = $function($router_path, $arg)) {
      $output .= $help . "\n";
    }
  }
  return $output;
}

Furthermore there is no cache clearing function served by calling drupal_help_arg() without an argument. The $empty_arg variable was used in Drupal 6 but is just cruft now.