[Memory-opti:fix leak] Fix WorldServer leak

[Alexdoru]

Turns out when you unregister a forge event class instance, that class instance is still referenced in forge's event system thus not getting garbage collected

So I manually clear the World map data when unregistering the event to avoid leaking the World object.

On top of that the mod used the FMLServerStoppingEvent to clear the data which fires too early, before the WorldEvent.Unload event fires on the server side. So by using the FMLServerStoppedEvent instead the event handler can now run properly and clear the world map.