Core/Gameobject: remove immune flags from trigger used to cast GO spells

Spells were failing flags check in Unit::_IsValidAttackTarget

Closes #18206

src/server/game/Entities/GameObject/GameObject.cpp

@@ -1886,6 +1886,9 @@ void GameObject::CastSpell(Unit* target, uint32 spellId, TriggerCastFlags trigge
     if (!trigger)
         return;
 
+    // remove immunity flags, to allow spell to target anything
+    trigger->RemoveFlag(UNIT_FIELD_FLAGS, UNIT_FLAG_IMMUNE_TO_NPC | UNIT_FLAG_IMMUNE_TO_PC);
+
     if (Unit* owner = GetOwner())
     {
         trigger->setFaction(owner->getFaction());