Scripts/MagistersTerrace: reworked InstanceScript and fixed some issues ...

...on boss Vexallus

* removed old hacks and added missing auras
* Vexallus converted to EventMap
i like more:

```c++
                     case GO_VEXALLUS_DOOR:
                     case GO_SELIN_DOOR:
                         AddDoor(go, true);
                         break;
                     case GO_SELIN_ENCOUNTER_DOOR:
                     case GO_DELRISSA_DOOR:
                     case GO_KAEL_DOOR:
                         AddDoor(go, true);
                         break;
```