[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/string b/include/string
index 4940021..01b79d8 100644
--- a/include/string
+++ b/include/string
@@ -1732,7 +1732,7 @@
 #endif
 
 
-#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
+#if _LIBCPP_STD_VER >= 17
 template<class _InputIterator,
          class _CharT = __iter_value_type<_InputIterator>,
          class _Allocator = allocator<_CharT>,