Remove redundant specializations

Author: vitaut

include/fmt/xchar.h

@@ -112,10 +112,6 @@ inline auto runtime(wstring_view s) -> runtime_format_string<wchar_t> {
   return {{s}};
 }
 
-template <> struct is_char<wchar_t> : std::true_type {};
-template <> struct is_char<char16_t> : std::true_type {};
-template <> struct is_char<char32_t> : std::true_type {};
-
 #ifdef __cpp_char8_t
 template <> struct is_char<char8_t> : bool_constant<detail::is_utf8_enabled> {};
 #endif