Remove all blimp network code.

blimp/net/blimp_message_checkpoint_observer.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 BLIMP_NET_BLIMP_MESSAGE_CHECKPOINT_OBSERVER_H_
-#define BLIMP_NET_BLIMP_MESSAGE_CHECKPOINT_OBSERVER_H_
-
-#include <stdint.h>
-
-namespace blimp {
-
-// Allows objects to subscribe to message acknowledgment checkpoints.
-class BlimpMessageCheckpointObserver {
- public:
-  virtual ~BlimpMessageCheckpointObserver() {}
-
-  // Invoked when the remote end has positively acknowledged the receipt of all
-  // messages with ID <= |message_id|.
-  virtual void OnMessageCheckpoint(int64_t message_id) = 0;
-};
-
-}  // namespace blimp
-
-#endif  // BLIMP_NET_BLIMP_MESSAGE_CHECKPOINT_OBSERVER_H_