[flang][nfc] Remove unused codes in idioms.h

Since D137859, these have not been used.

Reviewed By: Renaud-K

Differential Revision: https://reviews.llvm.org/D146709

flang/include/flang/Common/idioms.h

@@ -112,20 +112,6 @@ template <typename... LAMBDAS> visitors(LAMBDAS... x) -> visitors<LAMBDAS...>;
   } \
   template <typename A> constexpr bool T{class_trait_ns_##T::trait_value<A>()};
 
-// Define enum class NAME with the given enumerators,
-// - a static function EnumToString() that maps enumerators to std::string,
-// - a constant NAME_enumSize that captures the number of items in the enum,
-// - a struct NAME_struct that implements a Meyers singleton to hold the mapping
-// from index to names
-
-void BuildIndexToString(
-    const char *commaSeparated, std::string enumNames[], int enumSize);
-
-template <typename A> struct ListItemCount {
-  constexpr ListItemCount(std::initializer_list<A> list) : value{list.size()} {}
-  const std::size_t value;
-};
-
 // Check that a pointer is non-null and dereference it
 #define DEREF(p) Fortran::common::Deref(p, __FILE__, __LINE__)