bindings: Fix teardown of workers

The fix in https://crrev.com/de39db161c8f0e032094c3458d399191f116a4c2
had the right idea but missed out on removing the parts that clear the
references too eagerly.

This patch keeps the same fix but ensures that the old code is
removed.

I manually verified that this fixes a CF issue (1079033) that still
reproduces on master.

Bug: chromium:1078751, chromium:1079033
Change-Id: Iaddbbb17b360304c6c9b59399bc6e4e219833c47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2187269Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#766313}

third_party/blink/renderer/platform/bindings/v8_per_isolate_data.cc

@@ -165,8 +165,6 @@ void V8PerIsolateData::WillBeDestroyed(v8::Isolate* isolate) {
     data->profiler_group_ = nullptr;
   }
 
-  data->active_script_wrappable_manager_.Clear();
-
   // Detach V8's garbage collector.
   // Need to finalize an already running garbage collection as otherwise
   // callbacks are missing and state gets out of sync.