Remove unused type

include/fmt/base.h

@@ -2122,8 +2122,6 @@ auto get_iterator(buffer<T>&, OutputIt out) -> OutputIt {
 // This type is intentionally undefined, only used for errors.
 template <typename T, typename Char> struct type_is_unformattable_for;
 
-struct custom_tag {};
-
 template <typename Char> struct string_value {
   const Char* data;
   size_t size;