[promises] Clean up promise utils and remove dead code

src/promise-utils.h

-// 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.
-
-#ifndef V8_PROMISE_UTILS_H_
-#define V8_PROMISE_UTILS_H_
-
-#include "src/contexts.h"
-#include "src/objects.h"
-
-namespace v8 {
-namespace internal {
-
-// Helper methods for Promise builtins.
-class PromiseUtils : public AllStatic {
- public:
-  enum PromiseResolvingFunctionContextSlot {
-    // Whether the resolve/reject callback was already called.
-    kAlreadyVisitedSlot = Context::MIN_CONTEXT_SLOTS,
-
-    // The promise which resolve/reject callbacks fulfill.
-    kPromiseSlot,
-
-    // Whether to trigger a debug event or not. Used in catch
-    // prediction.
-    kDebugEventSlot,
-    kPromiseContextLength,
-  };
-
-  // These get and set the slots on the PromiseResolvingContext, which
-  // is used by the resolve/reject promise callbacks.
-  static JSPromise* GetPromise(Handle<Context> context);
-  static Object* GetDebugEvent(Handle<Context> context);
-  static bool HasAlreadyVisited(Handle<Context> context);
-  static void SetAlreadyVisited(Handle<Context> context);
-};
-
-class GetPromiseCapabilityExecutor : public AllStatic {
- public:
-  enum FunctionContextSlot {
-    kCapabilitySlot = Context::MIN_CONTEXT_SLOTS,
-
-    kContextLength,
-  };
-};
-
-}  // namespace internal
-}  // namespace v8
-
-#endif  // V8_PROMISE_UTILS_H_