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

[sunshaoce]

Summary

Since D137859, these have not been used.

[Renaud-K]

Pre-commits are failing because of https://reviews.llvm.org/D143704?id=507486, though they should have been fixed by now. Restarted build.
Otherwise it looks good this code was moved under enum-class.h.

[Renaud-K]

I cannot explain why pre-commits are still failing.

[sunshaoce]

Attempt to fix build error by using rebase.

[sunshaoce]

Rebase

flang/include/flang/Common/idioms.h

template <typename A> constexpr bool trait_value() { \
         return U::value; \
       } else { \
         return false; \
       } \
     } \
   } \
   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__)
 
 template <typename T> constexpr T &Deref(T *p, const char *file, int line) {
   if (!p) {
     Fortran::common::die("nullptr dereference at %s(%d)", file, line);
   }
   return *p;