Core/Creatures: remove virtual mounts from creatures on death

Closes https://github.com/TrinityCore/TrinityCore/pull/14396

src/server/game/Entities/Creature/Creature.cpp

@@ -1482,6 +1482,8 @@ void Creature::setDeathState(DeathState s)
         SetTarget(ObjectGuid::Empty);                // remove target selection in any cases (can be set at aura remove in Unit::setDeathState)
         SetUInt32Value(UNIT_NPC_FLAGS, UNIT_NPC_FLAG_NONE);
 
+        SetUInt32Value(UNIT_FIELD_MOUNTDISPLAYID, 0); // if creature is mounted on a virtual mount, remove it at death
+
         setActive(false);
 
         if (HasSearchedAssistance())