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

ipc/placeholder_brokerable_attachment.cc

Index: ipc/placeholder_brokerable_attachment.cc
diff --git a/ipc/placeholder_brokerable_attachment.cc b/ipc/placeholder_brokerable_attachment.cc
deleted file mode 100644
index e64e3985689a0b68b5a0eb7a70e31e5fef1cb412..0000000000000000000000000000000000000000
--- a/ipc/placeholder_brokerable_attachment.cc
+++ /dev/null
@@ -1,14 +0,0 @@
-// 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.
-
-#include "ipc/placeholder_brokerable_attachment.h"
-
-namespace IPC {
-
-BrokerableAttachment::BrokerableType
-PlaceholderBrokerableAttachment::GetBrokerableType() const {
-  return PLACEHOLDER;
-}
-
-}  // namespace IPC