v8binding: Removes unused EventListener::kNativeEventListenerType.

Change-Id: I00c7a48e7b978bf2a7d34d0deeb0c345b6e7f651
Reviewed-on: https://chromium-review.googlesource.com/1056899Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558237}

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

@@ -40,7 +40,6 @@ class CORE_EXPORT EventListener
     kImageEventListenerType,
     kCPPEventListenerType,
     kConditionEventListenerType,
-    kNativeEventListenerType,
   };
 
   virtual ~EventListener() = default;