Revert of IPC: Add attachment brokering support to the message header.

ipc/placeholder_brokerable_attachment.h

-// Copyright 2015 The Chromium 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 IPC_PLACEHOLDER_BROKERABLE_ATTACHMENT_H_
-#define IPC_PLACEHOLDER_BROKERABLE_ATTACHMENT_H_
-
-#include "ipc/brokerable_attachment.h"
-#include "ipc/ipc_export.h"
-
-namespace IPC {
-
-// This subclass of BrokerableAttachment has an AttachmentId, and nothing else.
-// It is intended to be replaced by the attachment broker.
-class IPC_EXPORT PlaceholderBrokerableAttachment : public BrokerableAttachment {
- public:
-  PlaceholderBrokerableAttachment(const AttachmentId& id)
-      : BrokerableAttachment(id){};
-  BrokerableType GetBrokerableType() const override;
-
- protected:
-  ~PlaceholderBrokerableAttachment() override{};
-  DISALLOW_COPY_AND_ASSIGN(PlaceholderBrokerableAttachment);
-};
-
-}  // namespace IPC
-
-#endif  // IPC_PLACEHOLDER_BROKERABLE_ATTACHMENT_H_