Remove MoveToNextSiblingItem from ng_inline_cursor.h

This is a left over from r762697 <crrev.com/c/2166202>.

This patch has no behavior changes.

982194, 1035773

Change-Id: I3b094e02b840c0f7bf36abaeb2ac38c8748abb03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2167635
Commit-Queue: Koji Ishii <kojii@chromium.org>
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762723}

third_party/blink/renderer/core/layout/ng/inline/ng_inline_cursor.h

@@ -420,7 +420,6 @@ class CORE_EXPORT NGInlineCursor {
   void MoveToItem(const ItemsSpan::iterator& iter);
   void MoveToNextItem();
   void MoveToNextItemSkippingChildren();
-  void MoveToNextSiblingItem();
   void MoveToPreviousItem();
 
   void MoveToParentPaintFragment();