AOI S3: Clear all enemies on victory

... for consistency with the previous message stating that
all orcs are killed by the elves.

data/campaigns/An_Orcish_Incursion/scenarios/03_Wasteland.cfg

@@ -222,6 +222,10 @@
             message= _ "The orcish chieftain finally fell, overcome by his wounds. The elves found themselves in a half-ruined camp littered with the bodies of both sides. There was no hale orc in sight, nor did any of the wounded beg mercy. The elves took no prisoners."
         [/message]
 
+        [kill]
+            side=2
+        [/kill]
+
         [endlevel]
             result=victory
             bonus=yes