Fixed killed entities not being removed from arrowTracker

src/main/java/com/gmail/nossr50/skills/archery/Archery.java

@@ -46,6 +46,7 @@ public class Archery {
 
         if (quantity != null) {
             Misc.dropItems(entity.getLocation(), new ItemStack(Material.ARROW), quantity);
+            arrowTracker.remove(entity);
         }
     }