TSG S4: minor optimizations

Made it impossible that the undead leader can advance to a lich - though you do
sth. badly wrong if he advances at all. Also, some more 1.13 syntactic sugar.

[ci skip]

data/campaigns/The_South_Guard/scenarios/04_Vale_of_Tears.cfg

@@ -63,6 +63,7 @@
 #endif
 #ifdef NORMAL
         type=Dark Sorcerer
+        advances_to=Necromancer
 #endif
 #ifdef HARD
         type=Necromancer
@@ -340,7 +341,7 @@
         [/filter_attack]
 
         [filter_second]
-            type=Skeleton, Revenant, Deathblade, Draug, Skeleton Archer, Bone Shooter, Banebow
+            type_adv_tree=Skeleton,Skeleton Archer
         [/filter_second]
 
         # only comment on pierce damage being useless if the player didn't