Core/Spawning: Actually check spawn group state before processing a respawn. It feels like that is something that should've been noticed at some point.

Also remove CreatureScript::CanSpawn since nobody uses it, and spawn groups do the same thing.

(cherry picked from commit 94b5d9bfa1)

src/server/game/Instances/InstanceScript.cpp

@@ -271,7 +271,7 @@ void InstanceScript::UpdateSpawnGroups()
         if (doSpawn)
             instance->SpawnGroupSpawn(groupId);
         else // otherwise, set it as inactive so it no longer respawns (but don't despawn it)
-            instance->SetSpawnGroupActive(groupId, false);
+            instance->SetSpawnGroupInactive(groupId);
     }
 }