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 rP25657

Patch by: phosit

Reviewed By: wraitii

Differential Revision: https://code.wildfiregames.com/D4831