Pet/Guardian AI hook re-organizing (#19824)

* Pet/Guardian AI hook re-organizing:
- Adjust OwnerAttacked/OwnerAttackedBy hooks on CreatureAI to fire for all owned units, not just player pets. This should allow guardians to more reliably recognize valid targets.
- Kill off the AttackedBy hook. While it was defined in CreatureAI.h as virtual, it was only ever invoked for player pets in specific situations. This makes it classic developer bait.
  - Adjust PetAI to use DamageTaken instead of AttackedBy.
  - Adjust behavior of AttackStart on PetAI to compensate.

src/server/scripts/EasternKingdoms/zone_wetlands.cpp

@@ -106,17 +106,6 @@ public:
                 summoned->AI()->AttackStart(player);
         }
 
-        void AttackedBy(Unit* pAttacker) override
-        {
-            if (me->GetVictim())
-                return;
-
-            if (me->IsFriendlyTo(pAttacker))
-                return;
-
-            AttackStart(pAttacker);
-        }
-
         void DamageTaken(Unit* /*pDoneBy*/, uint32& uiDamage) override
         {
             if (HealthBelowPct(20))