[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/tuple b/include/tuple
index e1019ef..2e339f8 100644
--- a/include/tuple
+++ b/include/tuple
@@ -1097,7 +1097,7 @@
     void swap(tuple&) _NOEXCEPT {}
 };
 
-#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
+#if _LIBCPP_STD_VER >= 17
 template <class ..._Tp>
 tuple(_Tp...) -> tuple<_Tp...>;
 template <class _Tp1, class _Tp2>