Remove Event::IsRelatedEvent()

This CL is just a pure refactoring. RelatedEvent was removed at
https://codereview.chromium.org/2905763003.

Change-Id: Ib0c4a31944075a9361618ec556544a97bbb95cc0
Reviewed-on: https://chromium-review.googlesource.com/1096732Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566771}

third_party/blink/renderer/core/dom/events/event.cc

@@ -200,10 +200,6 @@ bool Event::IsWheelEvent() const {
   return false;
 }
 
-bool Event::IsRelatedEvent() const {
-  return false;
-}
-
 bool Event::IsPointerEvent() const {
   return false;
 }

third_party/blink/renderer/core/dom/events/event.h

@@ -176,7 +176,6 @@ class CORE_EXPORT Event : public ScriptWrappable {
   virtual bool IsTouchEvent() const;
   virtual bool IsGestureEvent() const;
   virtual bool IsWheelEvent() const;
-  virtual bool IsRelatedEvent() const;
   virtual bool IsPointerEvent() const;
   virtual bool IsInputEvent() const;
   virtual bool IsCompositionEvent() const;