Remove istring::append with template

include/mtl/string.hpp

@@ -99,9 +99,6 @@ public:
 	istring& append(const string_view& str);
 	istring& append(const string_view& str, size_t str_index, size_t count = npos);
 	istring& append(size_t count, char ch);
-	template <typename T>
-	istring& append(T first, T last) {
-	}
 
 	istring& operator+=(char ch);
 	istring& operator+=(const string_view& str);