[future.syn] Remove uses_allocator also from header synopsis.

P3503R3 didn't actually say this, but it is clearly necessary.

Author: tkoeppe

source/threads.tex

@@ -11183,9 +11183,6 @@
   template<class R>
     void swap(promise<R>& x, promise<R>& y) noexcept;
 
-  template<class R, class Alloc>
-    struct uses_allocator<promise<R>, Alloc>;
-
   // \ref{futures.unique.future}, class template \tcode{future}
   template<class R> class future;
   template<class R> class future<R&>;