Scripts/CoS: removed SetDespawnAtFar, fixed situation when Artas attacked some NPC and disappeared because he was out of range of players

With these lines it's almost impossible to finish properly this dungeon. For example when Arthas moves ahead to attack NPC and then returns back, when this happens, Arthas is despawned.

(cherry picked from commit 9cd59c258cdb455850a250842651c426dac36f05)