[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/array b/include/array
index 022172b..b76b9fa 100644
--- a/include/array
+++ b/include/array
@@ -356,7 +356,7 @@
 };
 
 
-#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
+#if _LIBCPP_STD_VER >= 17
 template<class _Tp, class... _Args,
          class = _EnableIf<__all<_IsSame<_Tp, _Args>::value...>::value>
          >