[libc++][NFC] Remove _VSTD:: when not needed.

Reviewed By: #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D102133

NOKEYCHECK=True
GitOrigin-RevId: cfef7c918b8297ffb1d882d6b31fe68f876607db
diff --git a/include/functional b/include/functional
index 19d74a3..3d8138a 100644
--- a/include/functional
+++ b/include/functional
@@ -2050,7 +2050,7 @@
 // faster for types that can be passed in registers.
 template <typename _Tp>
 using __fast_forward =
-    typename _VSTD::conditional<_VSTD::is_scalar<_Tp>::value, _Tp, _Tp&&>::type;
+    typename conditional<is_scalar<_Tp>::value, _Tp, _Tp&&>::type;
 
 // __policy_invoker calls an instance of __alloc_func held in __policy_storage.