Mojo: Remove MessagePipe::OnRemove(); instead just use MessagePipe::Close().

mojo/system/message_pipe_endpoint.cc

 // Copyright 2013 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 "mojo/system/message_pipe_endpoint.h"
 
 #include "base/logging.h"
 
 namespace mojo {
 namespace system {
 
 void MessagePipeEndpoint::Close() {
-  NOTREACHED();
 }
 
 void MessagePipeEndpoint::CancelAllWaiters() {
   NOTREACHED();
 }
 
 MojoResult MessagePipeEndpoint::ReadMessage(UserPointer<void> /*bytes*/,
                                             UserPointer<uint32_t> /*num_bytes*/,
                                             DispatcherVector* /*dispatchers*/,
                                             uint32_t* /*num_dispatchers*/,
@@ skipping 23 matching lines @@
   if (signals_state)
     *signals_state = HandleSignalsState();
 }
 
 void MessagePipeEndpoint::Attach(ChannelEndpoint* /*channel_endpoint*/) {
   NOTREACHED();
 }
 
 }  // namespace system
 }  // namespace mojo