Gadget for removing XP gains from killing blows

removes the xp units gain for placing the final shot on something. tested this and it appears to work fine.

- same team kills cause the function to return (no xp is added for this from the engine either)
- nil attackers cause the function to return
- there are no other safeguards against funky attackerIDs or unitIDs
- xp losses from this gadget will be delayed to the next game frame if xp would drop below zero (which they cant)


there is one edge case i couldnt test: if a unit was to kill multiple units in the same frame without gaining substantial xp from the damage done (such as oneshotting 10 detriments that are each at 1 hp), it should add the amount to be substracted up and then remove it all at the end of frame.