Deprecate Kokkos::is_reducer_type

[dalg24]

Rational: obscure duplicate functionality

Haven't found any usage in Trilinos. I suggest we deprecate/remove.

[dalg24]

For reference is_reducer is defined here

kokkos/core/src/Kokkos_Concepts.hpp

Line 174 in 02ba049

KOKKOS_IMPL_IS_CONCEPT(reducer)

which is expanded using

kokkos/core/src/Kokkos_Concepts.hpp

Lines 151 to 165 in 02ba049

	#define KOKKOS_IMPL_IS_CONCEPT(CONCEPT) \
	template <typename T> \
	struct is_##CONCEPT { \
	private: \
	template <typename U> \
	using have_t = typename U::CONCEPT; \
	template <typename U> \
	using have_type_t = typename U::CONCEPT##_type; \
	\
	public: \
	static constexpr bool value = \
	std::is_base_of<detected_t<have_t, T>, T>::value || \
	std::is_base_of<detected_t<have_type_t, T>, T>::value; \
	constexpr operator bool() const noexcept { return value; } \
	};