fix advancement for Darken Volk (#6358)

* fix advancement for Darken Volk

* moe modify_unit_type in [campaign]

* revert eb6eb7d962

data/campaigns/Descent_Into_Darkness/_main.cfg

@@ -15,6 +15,10 @@
     define=CAMPAIGN_DESCENT
     first_scenario=01_Saving_Parthyn
 
+    [modify_unit_type]
+        type="Dark Sorcerer"
+        remove_advancement="Lich"
+    [/modify_unit_type]
     {CAMPAIGN_DIFFICULTY EASY   "units/undead-necromancers/adept.png~RC(magenta>black)" ( _ "Neophyte") ( _ "Easy")}
     {CAMPAIGN_DIFFICULTY NORMAL "units/undead-necromancers/dark-sorcerer.png~RC(magenta>black)" ( _ "Evoker") ( _ "Normal")} {DEFAULT_DIFFICULTY}
     {CAMPAIGN_DIFFICULTY HARD   "units/undead-necromancers/lich.png~RC(magenta>black)" ( _ "Summoner") ( _ "Challenging")}

data/campaigns/Descent_Into_Darkness/scenarios/08_Alone_at_Last.cfg

@@ -193,6 +193,11 @@
             x,y=2,26
         [/recall]
 
+        [remove_trait]
+            id=Darken Volk
+            trait_id="loyal"
+        [/remove_trait]
+
         [modify_unit]
             [filter]
                 id=Darken Volk

data/campaigns/Descent_Into_Darkness/utils/characters.cfg

@@ -35,9 +35,10 @@
     name= _ "Darken Volk"
     profile=portraits/darken_volk.png
     unrenamable=yes
-    advances_to=Necromancer
     random_traits=no
     [modifications]
+        # NOTE: if you change these, make sure you don't break the
+        # removal of the loyal trait in 'Alone at Last
         [trait]
             id=loyal
             name= _ "loyal"