arc-notifications: Remove obsolete comment.

Test: none
Bug: none
Change-Id: Id881b441d8ef888a901eda05a02438790df439bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507245
Commit-Queue: Sara Kato <sarakato@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823035}

ash/public/cpp/external_arc/message_center/arc_notification_manager.cc

@@ -205,7 +205,6 @@ void ArcNotificationManager::OnNotificationPosted(ArcNotificationDataPtr data) {
     // TODO(yoshiki): Reconsider when ARC supports multi-user.
     auto item = std::make_unique<ArcNotificationItemImpl>(
         this, message_center_, key, main_profile_id_);
-    // TODO(yoshiki): Use emplacement for performance when it's available.
     auto result = items_.insert(std::make_pair(key, std::move(item)));
     DCHECK(result.second);
     it = result.first;