Revert "Add ChannelEndpoint::ReplaceClient()."

This reverts commit 505555fa8031fc06d67e9ed43e3021d8d84cb44f.
(And also 8599bcc2cb49a406e6394077ba972b8311a53f7c.)

Fails on Windows for some reason:

[ RUN      ] RemoteMessagePipeTest.RacingClosesStress
[3476:752:1202/175739:1041712:FATAL:channel_endpoint.cc(166)] Check failed: is_detached_from_channel_.
Backtrace:
base::debug::StackTrace::StackTrace [0x000000014027FFF6+54] (...\src\base\debug\stack_trace_win.cc:209)
logging::LogMessage::~LogMessage [0x000000014032C0DD+77] (...\src\base\logging.cc:546)
mojo::system::ChannelEndpoint::DetachFromChannel [0x00000001400D6100+944] (...\src\mojo\edk\system\channel_endpoint.cc:168)
mojo::system::Channel::OnRemoveMessagePipeEndpoint [0x00000001401D6ED7+903] (...\src\mojo\edk\system\channel.cc:470)
mojo::system::Channel::OnReadMessageForChannel [0x00000001401D5A96+1590] (...\src\mojo\edk\system\channel.cc:373)
mojo::system::Channel::OnReadMessage [0x00000001401D534C+492] (...\src\mojo\edk\system\channel.cc:244)
mojo::system::RawChannel::OnReadCompleted [0x0000000140230D2A+2298] (...\src\mojo\edk\system\raw_channel.cc:362)
mojo::system::`anonymous namespace'::RawChannelWin::RawChannelIOHandler::OnReadCompleted [0x0000000140238502+914] (...\src\mojo\edk\system\raw_channel_win.cc:308)
mojo::system::`anonymous namespace'::RawChannelWin::RawChannelIOHandler::OnIOCompleted [0x00000001402374BD+397] (...\src\mojo\edk\system\raw_channel_win.cc:258)
base::MessagePumpForIO::WaitForIOCompletion [0x000000014037BCE8+536] (...\src\base\message_loop\message_pump_win.cc:544)
...

TBR=yzshen@chromium.org

Review URL: https://codereview.chromium.org/771353003