remove the unused traits and add documentation

Co-authored-by: Marcel Koch <marcel.koch@kit.edu>

Author: yhmtsai

include/ginkgo/core/base/type_traits.hpp

@@ -34,15 +34,6 @@ template <typename Type>
 using factory_type = typename factory_type_impl<Type>::type;
 
 
-template <typename Type>
-struct get_solver_type_impl {
-    using type = std::conditional_t<is_ginkgo_linop<Type>, Type, LinOp>;
-};
-
-template <typename Type>
-using get_solver_type = typename get_solver_type_impl<Type>::type;
-
-
 // helper for handle the transposed type of concrete type and LinOp
 template <typename Type>
 struct transposed_type_impl {

include/ginkgo/core/preconditioner/ic.hpp

@@ -148,6 +148,11 @@ class Ic : public EnableLinOp<Ic<LSolverTypeOrValueType, IndexType>>,
             return with_l_solver(std::move(solver));
         }
 
+        /**
+         * When LSolverTypeOrValueType is a concrete solver type, this only
+         * accepts the factory from the same concrete solver type. When
+         * LSolverTypeOrValueType is a value type, it accepts any LinOpFactory.
+         */
         parameters_type& with_l_solver(
             deferred_factory_parameter<
                 const gko::detail::factory_type<l_solver_type>>