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

Differential Revision: https://reviews.llvm.org/D138687

libcxx/include/__thread/poll_with_backoff.h

@@ -13,10 +13,7 @@
 #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

libcxx/include/module.modulemap.in

@@ -672,6 +672,7 @@ module std [system] {
         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" }