Remove calls to deprecated global $user in translation module

Part of #2047951: [META] Remove calls to deprecated global $user and $GLOBALS['user']

core/modules/translation/translation.module:

function translation_user_can_translate_node($node, $account = NULL) {
  // If no user object is supplied, the access check is for the current user.
  if (empty($account)) {
    $account = $GLOBALS['user'];
  }