Fix cleanup of old units

[Tuupertunut]

Cleanup of old units (like a patrol after it finished and returned to base) was apparently very broken. Now it works correctly for groups, units, vehicles and their crews. This might improve performance a lot when the game has been going on for a long time.

Bugs fixed:

• Groups were never cleaned up
• Vehicle crew was sometimes not cleaned up
• Helicopters were sometimes not cleaned up

More info in the commit messages.

[rekterakathom]

Even though I am not entirely sure that this is the best practice, I am going to merge this and it can be looked into later. Cleanup is important and we just need something that works instead of waiting around for the optimal solution to be found.