[libc++][NFC] Remove unused includes from poll_with_backoff.h

[ldionne]

Fix modules issue

libcxx/include/__thread/poll_with_backoff.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
 #define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
 
 #include <__availability>
 #include <__chrono/duration.h>
 #include <__chrono/high_resolution_clock.h>
-#include <__chrono/steady_clock.h>
-#include <__chrono/time_point.h>
 #include <__config>
-#include <__filesystem/file_time_type.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;

libcxx/include/module.modulemap.in

module __chrono {
         @requires_LIBCXX_ENABLE_LOCALIZATION@
         private header "__chrono/formatter.h"
       }
       module hh_mm_ss               { private header "__chrono/hh_mm_ss.h" }
       module high_resolution_clock  {
         private header "__chrono/high_resolution_clock.h"
         export steady_clock
         export system_clock
+        export time_point
       }
       module literals               { private header "__chrono/literals.h" }
       module month                  { private header "__chrono/month.h" }
       module month_weekday          { private header "__chrono/month_weekday.h" }
       module monthday               { private header "__chrono/monthday.h" }
       module ostream                {
         @requires_LIBCXX_ENABLE_LOCALIZATION@
         private header "__chrono/ostream.h"