Disable standard advancement to Khalid

Require the {ENABLE_KHALID} macro to allow Shuja to advance to Khalid.
Khalid are described as legendary heroes, not the meat of your late-game
army. Thus, the L4 is handled similar to the arcanister, armageddon drake
and ancient lich.

[ci skip]

data/core/units/khalifate/Khalid.cfg

@@ -12,6 +12,12 @@
     level=4
     alignment=lawful
     advances_to=null
+#ifdef ENABLE_KHALID
+    [advancefrom]
+        unit=Shuja
+        experience=150
+    [/advancefrom]
+#endif
     {AMLA_DEFAULT}
     cost=78
     usage=fighter

data/core/units/khalifate/Shuja.cfg

@@ -11,7 +11,7 @@
     experience=150
     level=3
     alignment=lawful
-    advances_to=Khalid
+    advances_to=null
     cost=58
     usage=fighter
     description= _ "Distinguished for their bravery and skill, Shujas are the leaders among the Khalif's ranks. Their exploits are well known among their cohorts, inspiring soldiers to fight for their cause. Despite this, they are often found leading from the front, taking down their foes with sword and shield."+{SPECIAL_NOTES}+{SPECIAL_NOTES_MARKSMAN}+{SPECIAL_NOTES_SLOW}