updated World.deleteEntity to always try to remove the entity from the "added" bag.

This fixes a lot of NPEs when an entity is added and deleted very quickly.

I am in the process of taking the old SpaceshipWarrior artemis project and "modernizing" it with a full maven libgdx project using artemis-odb. I was getting lots of NullPointerExceptions  when the bullets were deleted very quickly (when directly on top of a ship and firing).

I tracked it down to this little change. Please let me know what you think.