Use Future::CancelOrWait in pathfinder Deinit

If the pathfinding tasks are running, they might reference now-deleted
variables. CancelOrWait prevents this.
Remove `Future::Cancel()` altogether as that was its only use and the
functions seems dangerous.

Introduced with 0ebc08b13c

Patch by: phosit
Reviewed By: wraitii
Differential Revision: https://code.wildfiregames.com/D4831
This was SVN commit r27310.

source/ps/Future.h

@@ -222,17 +222,6 @@ public:
 		m_SharedState.reset();
 	}
 
-	/**
-	 * Cancels the task (without waiting).
-	 * The result is always invalid, even if the task had completed before.
-	 * Note that this cannot stop started tasks.
-	 */
-	void Cancel()
-	{
-		if (m_SharedState)
-			m_SharedState->Cancel();
-		m_SharedState.reset();
-	}
 protected:
 	std::shared_ptr<SharedState> m_SharedState;
 };

source/ps/tests/test_Future.h

@@ -95,8 +95,7 @@ public:
 		{
 			Future<NonDef> future;
 			std::function<void()> task = future.Wrap([]() { return 1; });
-			future.Cancel();
-			future.Wait();
+			future.CancelOrWait();
 			TS_ASSERT_THROWS(future.Get(), const Future<NonDef>::BadFutureAccess&);
 		}
 		 */

source/simulation2/components/CCmpPathfinder.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2021 Wildfire Games.
+/* Copyright (C) 2022 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -107,7 +107,7 @@ void CCmpPathfinder::Deinit()
 
 	// Wait on all pathfinding tasks.
 	for (Future<void>& future : m_Futures)
-		future.Cancel();
+		future.CancelOrWait();
 	m_Futures.clear();
 
 	SAFE_DELETE(m_AtlasOverlay);