[libc++] Remove workarounds for the lack of deduction guides in C++17

All supported compilers have supported deduction guides in C++17 for a
while, so this isn't necessary anymore.

Differential Revision: https://reviews.llvm.org/D108213

NOKEYCHECK=True
GitOrigin-RevId: 01666904013c1514b16ace896ba346d1273a034f
diff --git a/include/stack b/include/stack
index aefef31..5ee4d51 100644
--- a/include/stack
+++ b/include/stack
@@ -231,7 +231,7 @@
     operator< (const stack<T1, _C1>& __x, const stack<T1, _C1>& __y);
 };
 
-#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
+#if _LIBCPP_STD_VER >= 17
 template<class _Container,
          class = _EnableIf<!__is_allocator<_Container>::value>
 >