Revert "flang: Fix flang build with -Wctad-maybe-unsupported"

-Wctad-maybe-unsupported is now disabled for flang so these explicit
deduction guides are not required.

This reverts commit 248591aabe.

flang/include/flang/Common/idioms.h

@@ -123,9 +123,6 @@ template <typename A> struct ListItemCount {
   const std::size_t value;
 };
 
-template<typename A>
-ListItemCount(std::initializer_list<A>) -> ListItemCount<A>;
-
 #define ENUM_CLASS(NAME, ...) \
   enum class NAME { __VA_ARGS__ }; \
   LLVM_ATTRIBUTE_UNUSED static constexpr std::size_t NAME##_enumSize{[] { \