AnyUnitsAt() : Ignore Units which are not in world and Destroyed

Fixes  #6883

i hope this fix doesnt breaks anything other :(
You are right. Also https://github.com/OpenRA/OpenRA/issues/6883 only crashed because because of a trait lookup from a destroyed object and it will never check for available sub-cells when the unit is not in the world. Just remove `i.Actor.IsInWorld &&` then @LipkeGu.