Changes

#1272: Only allow one entity in CraftItem instead of two

This pr reverts a change made in 2012 which allowed CraftItem to be a also other entities such as an arrow.
I don't see any practical reason why the original change was done, other than to allow plugin developers to do some stuff in a cursed way.
With the introduction of the PlayerPickupArrowEvent in 2016 there is also a way to access the arrow involved in a better way.
Generally I think the current behavior is a bit strange and should be reverted.