[libc++] Remove more unnecessary _VSTD:: from type names. NFCI.

Differential Revision: https://reviews.llvm.org/D102181

NOKEYCHECK=True
GitOrigin-RevId: 6491d99e330c38b33b9cb6acb19afa3a464febeb
diff --git a/include/memory b/include/memory
index 9b01b84..5d53ea5 100644
--- a/include/memory
+++ b/include/memory
@@ -863,7 +863,7 @@
 #endif
 {
     _VSTD::__swap_allocator(__a1, __a2,
-      integral_constant<bool, _VSTD::allocator_traits<_Alloc>::propagate_on_container_swap::value>());
+      integral_constant<bool, allocator_traits<_Alloc>::propagate_on_container_swap::value>());
 }
 
 template <typename _Alloc, typename _Traits=allocator_traits<_Alloc> >