Package and city marker clean-up

[Pattrigue]

This is related to issue https://github.com/SebLague/Geographical-Adventures/issues/35. I implemented a primitive system that destroys the previous package and city marker when a new package is dropped to prevent more and more GameObjects from building up over time (my poor memory!).

I moved the packageDropped event from the player into the quest system since it was unused anyway, and also the quest system is responsible for package dropping, not the player.

If this causes merge conflicts, you could discard the change I made to the Game.unity scene since you just have to attach the PackageCleanup component to the Quest System GameObject for the clean-up system to work.

A possible extra detail/improvement could be old packages and city markers fading out before getting destroyed.