Don't execute the task when no Future awaits it anymore

Summary:
Most of the times the callback stores a reference to a variable in scope
where the `Future` is in. When the scope is left the reference get's
dangling. `CancelOrWait` is called in multiple places (mostly
destructors) to ensure the callback isn't executed anymore.
This patch deduplicates thous calls to `CancelOrWait`.

Refs: #5874

Comments by: @Stan, @vladislavbelov
Differential Revision: https://code.wildfiregames.com/D5208
This was SVN commit r28128.

source/graphics/MapReader.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2023 Wildfire Games.
+/* Copyright (C) 2024 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -1331,11 +1331,6 @@ struct CMapReader::GeneratorState
 {
 	std::atomic<int> progress{1};
 	Future<Script::StructuredClone> task;
-
-	~GeneratorState()
-	{
-		task.CancelOrWait();
-	}
 };
 
 int CMapReader::StartMapGeneration(const CStrW& scriptFile)