cut c++17 guards around imports/backports of in-place tags

Summary: We have C++17 and C++17 has std::in_place etc.

Reviewed By: ot

Differential Revision: D55038202

fbshipit-source-id: 4922c9c3bef617c3391b84efa8d52857ede3873c

folly/Utility.h

@@ -165,8 +165,6 @@ constexpr like_t<Src, Dst>&& forward_like(Dst&& dst) noexcept {
  *    std::in_place_index
  */
 
-#if FOLLY_CPLUSPLUS >= 201703L
-
 using std::in_place_t;
 
 using std::in_place_type_t;
@@ -179,29 +177,6 @@ using std::in_place_type;
 
 using std::in_place_index;
 
-#else
-
-struct in_place_t {
-  explicit in_place_t() = default;
-};
-FOLLY_INLINE_VARIABLE constexpr in_place_t in_place{};
-
-template <class>
-struct in_place_type_t {
-  explicit in_place_type_t() = default;
-};
-template <class T>
-FOLLY_INLINE_VARIABLE constexpr in_place_type_t<T> in_place_type{};
-
-template <std::size_t>
-struct in_place_index_t {
-  explicit in_place_index_t() = default;
-};
-template <std::size_t I>
-FOLLY_INLINE_VARIABLE constexpr in_place_index_t<I> in_place_index{};
-
-#endif
-
 /**
  * Initializer lists are a powerful compile time syntax introduced in C++11
  * but due to their often conflicting syntax they are not used by APIs for