[libc++] Remove invalid use of `#if _LIBCPP_STD_VER >= 11`, as `_LIBCPP_STD_VER`...

The relevant part of `__config` is:
```
#ifndef _LIBCPP_STD_VER
#  if  __cplusplus <= 201103L
#    define _LIBCPP_STD_VER 11
```

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D93025

diff --git a/libcxx/include/atomic b/libcxx/include/atomic
index 5994b0c..0fc799a 100644 (file)
--- a/libcxx/include/atomic
+++ b/libcxx/include/atomic
@@ -1503,8 +1503,6 @@ struct __cxx_atomic_impl : public _Base {
     using __cxx_contention_t = int64_t;
 #endif //__linux__
 
-#if _LIBCPP_STD_VER >= 11
-
 using __cxx_atomic_contention_t = __cxx_atomic_impl<__cxx_contention_t>;
 
 #ifndef _LIBCPP_HAS_NO_PLATFORM_WAIT
@@ -1582,8 +1580,6 @@ _LIBCPP_INLINE_VISIBILITY bool __cxx_atomic_wait(_Atp* __a, _Tp const __val, mem
     return __cxx_atomic_wait(__a, __test_fn);
 }
 
-#endif //_LIBCPP_STD_VER >= 11
-
 // general atomic<T>
 
 template <class _Tp, bool = is_integral<_Tp>::value && !is_same<_Tp, bool>::value>