Take advantage of C++17 features (#8858)

---------

Co-authored-by: Hyunsu Philip Cho <chohyu01@cs.washington.edu>
Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>

src/common/common.h

@@ -195,31 +195,6 @@ template <typename Indexable>
 XGBOOST_DEVICE size_t LastOf(size_t group, Indexable const &indptr) {
   return indptr[group + 1] - 1;
 }
-
-/**
- * \brief A CRTP (curiously recurring template pattern) helper function.
- *
- * https://www.fluentcpp.com/2017/05/19/crtp-helper/
- *
- * Does two things:
- * 1. Makes "crtp" explicit in the inheritance structure of a CRTP base class.
- * 2. Avoids having to `static_cast` in a lot of places.
- *
- * \tparam T The derived class in a CRTP hierarchy.
- */
-template <typename T>
-struct Crtp {
-  T &Underlying() { return static_cast<T &>(*this); }
-  T const &Underlying() const { return static_cast<T const &>(*this); }
-};
-
-/**
- * \brief C++17 std::as_const
- */
-template <typename T>
-typename std::add_const<T>::type &AsConst(T &v) noexcept {  // NOLINT(runtime/references)
-  return v;
-}
 }  // namespace common
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_COMMON_H_