Core/Creatures: Only remove formation movement on formation remove instead of fully resetting MotionMaster

Port From (https://github.com/TrinityCore/TrinityCore/commit/67dd9a920fdabb26e6429432885345b1a1771b9a)

Source/Game/Entities/Creature/CreatureGroups.cs

@@ -247,7 +247,7 @@ namespace Game.Entities
             foreach (var (creature, _) in _members)
             {
                 if (dismiss)
-                    creature.GetMotionMaster().Initialize();
+                    creature.GetMotionMaster().Remove(MovementGeneratorType.Formation, MovementSlot.Default);
                 else
                     creature.GetMotionMaster().MoveIdle();
                 Log.outDebug(LogFilter.Unit, $"CreatureGroup::FormationReset: Set {(dismiss ? "default" : "idle")} movement for member {creature.GetGUID()}");

Source/Game/Entities/Unit/Unit.cs

@@ -1067,12 +1067,13 @@ namespace Game.Entities
                     return;
                 }
 
-                if (vehicle.GetBase().IsCreature())
+                Creature vehicleBaseCreature = vehicle.GetBase().ToCreature();
+                if (vehicleBaseCreature != null)
                 {
                     // If a player entered a vehicle that is part of a formation, remove it from said formation
-                    CreatureGroup creatureGroup = vehicle.GetBase().ToCreature().GetFormation();
+                    CreatureGroup creatureGroup = vehicleBaseCreature.GetFormation();
                     if (creatureGroup != null)
-                        creatureGroup.RemoveMember(vehicle.GetBase().ToCreature());
+                        FormationMgr.RemoveCreatureFromGroup(creatureGroup, vehicleBaseCreature);
                 }
             }