[libc++] Remove <chrono> include from <future>

Reviewed By: Quuxplusone, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D120436

Author: philnik777

libcxx/include/future

@@ -377,8 +377,6 @@ template <class R, class Alloc> struct uses_allocator<packaged_task<R>, Alloc>;
 #include <thread>
 #include <version>
 
-#include <chrono> // TODO: Remove unused header
-
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif