ResourceManager doesn't implement Reference Counting.

According to [04_resource_management.adoc](https://github.com/KhronosGroup/Vulkan-Tutorial/blob/main/en/Building_a_Simple_Engine/Engine_Architecture/04_resource_management.adoc),

> When designing a resource management system, you’ll need to address several challenges:
> 3. Reference Counting - Track how many objects are using a resource to know when it can be safely unloaded.

This seems to be implemented as `int ResourceManager::ResourceData::refCount`, but it's never modified in [04_resource_management.adoc](https://github.com/KhronosGroup/Vulkan-Tutorial/blob/main/en/Building_a_Simple_Engine/Engine_Architecture/04_resource_management.adoc), and doesn't exist in [resource_manager.h](https://github.com/KhronosGroup/Vulkan-Tutorial/blob/main/attachments/simple_engine/resource_manager.h).