Core/Achievements: Fix ACHIEVEMENT_CRITERIA_TYPE_GET_KILLING_BLOWS

Confirmed in http://www.wowhead.com/achievement=3856#comments
http://www.wowhead.com/achievement=2910
http://www.wowhead.com/achievement=2149

And others with type ACHIEVEMENT_CRITERIA_TYPE_GET_KILLING_BLOW
Previous code would often give the achievement credit to the loot recipient, rather than the actual killer