Remove obsolete FIXME in flat_tree_traversal.h

https://bugs.webkit.org/show_bug.cgi?id=82702 was already closed. We can remove this old FIXME.

TBR=rakina

Change-Id: Ib04660f4cb4be99c68a71d935de4d8bf429716ce
Reviewed-on: https://chromium-review.googlesource.com/1163343Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580813}

third_party/blink/renderer/core/dom/flat_tree_traversal.h

@@ -49,9 +49,6 @@ bool CanBeDistributedToV0InsertionPoint(const Node& node);
 // |InsertionPoint::isActive()| for details of active insertion points, since
 // they aren't appeared in the flat tree. |assertPrecondition()| and
 // |assertPostCondition()| check this condition.
-//
-// FIXME: Make some functions inline to optimise the performance.
-// https://bugs.webkit.org/show_bug.cgi?id=82702
 class CORE_EXPORT FlatTreeTraversal {
   STATIC_ONLY(FlatTreeTraversal);