remove obsolete code

Now that `find_object` isn't defined in the header anymore, it doesn't
make sense to fiddle around with dependent names.

src/world.cpp

@@ -281,9 +281,7 @@ template<typename T>
 std::shared_ptr<T> world::find_object(object_id id)
 {
     static_assert(std::is_base_of_v<object, T>);
-    // make it a dependent name so that including "src/object.hpp" isn't needed
-    using U = std::conditional_t<std::is_same_v<T, object>, T, object>;
-    if (std::shared_ptr<U> ptr = find_object_(id); !ptr)
+    if (std::shared_ptr<object> ptr = find_object_(id); !ptr)
         return {};
     else if constexpr(std::is_same_v<T, object>)
         return ptr;