[promises] Clean up promise utils and remove dead code

src/promise-utils.cc

-// Copyright 2016 the V8 project authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "src/promise-utils.h"
-
-#include "src/factory.h"
-#include "src/isolate.h"
-#include "src/objects-inl.h"
-
-namespace v8 {
-namespace internal {
-
-JSPromise* PromiseUtils::GetPromise(Handle<Context> context) {
-  return JSPromise::cast(context->get(kPromiseSlot));
-}
-
-Object* PromiseUtils::GetDebugEvent(Handle<Context> context) {
-  return context->get(kDebugEventSlot);
-}
-
-bool PromiseUtils::HasAlreadyVisited(Handle<Context> context) {
-  return Smi::cast(context->get(kAlreadyVisitedSlot))->value() != 0;
-}
-
-void PromiseUtils::SetAlreadyVisited(Handle<Context> context) {
-  context->set(kAlreadyVisitedSlot, Smi::FromInt(1));
-}
-
-}  // namespace internal
-}  // namespace v8