[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/vector b/include/vector
index 9189ed4..3afaddc 100644
--- a/include/vector
+++ b/include/vector
@@ -931,7 +931,7 @@
   }
 };
 
-#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
+#if _LIBCPP_STD_VER >= 17
 template<class _InputIterator,
          class _Alloc = allocator<__iter_value_type<_InputIterator>>,
          class = _EnableIf<__is_allocator<_Alloc>::value>