Do not clear TraceWrapperV8Reference in blink::V8AbstractEventListener destructor

This CL removed some code that clearing TraceWrapperV8Reference object
in its destructor.
|listener_| is cleared in the destructor of TraceWrapperV8Reference, so
we do not have to clear it in blink::V8AbstractEventListener.

Bug: 872138
Change-Id: I4f8e8e67074ee8b2081735053b5b0f7604afadff
Reviewed-on: https://chromium-review.googlesource.com/1173195
Commit-Queue: Yuki Yamada <yukiy@google.com>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582819}

third_party/blink/renderer/bindings/core/v8/v8_abstract_event_listener.cc

@@ -70,8 +70,6 @@ V8AbstractEventListener::~V8AbstractEventListener() {
     DCHECK(listener_.IsEmpty());
     InstanceCounters::DecrementCounter(
         InstanceCounters::kJSEventListenerCounter);
-  } else {
-    listener_.Clear();
   }
 }