Core/SAI: Rename LeavingWorld to OnDespawn & use it in SAI (#28059)

src/server/game/Entities/Unit/Unit.cpp

@@ -9584,7 +9584,7 @@ void Unit::RemoveFromWorld()
     {
         m_duringRemoveFromWorld = true;
         if (UnitAI* ai = GetAI())
-            ai->LeavingWorld();
+            ai->OnDespawn();
 
         if (IsVehicle())
             RemoveVehicleKit();