Core/Spells: Allow AoE spells to target dead units.

Also removes some unneeded checks from ally targeting.

--HG--
branch : trunk

src/server/game/Entities/Unit/Unit.h

@@ -1413,7 +1413,7 @@ class Unit : public WorldObject
         bool isFrozen() const;
 
         bool isTargetableForAttack() const;
-        bool isAttackableByAOE() const;
+        bool isAttackableByAOE(bool requireDeadTarget = false) const;
         bool canAttack(Unit const* target, bool force = true) const;
         virtual bool IsInWater() const;
         virtual bool IsUnderWater() const;