Comment out references to nonexistent FullScreen theme,

it was crashing the engine.  Works around bug #14165 in mainline, but

(a) The WML engine should complain on a noexistent theme and then
fall back to the default, rather than puking its guts out.

(a) FullScreen probably should come back as a feature.

data/campaigns/Heir_To_The_Throne/scenarios/21_Elven_Council.cfg

@@ -5,7 +5,7 @@
     map_data="{campaigns/Heir_To_The_Throne/maps/21_Elven_Council.map}"
 
     turns=1
-    theme=FullScreen
+    #theme=FullScreen
     {SCENARIO_MUSIC "revelation.ogg"}
 
     next_scenario=22_Return_to_Wesnoth

data/campaigns/Legend_of_Wesmere/scenarios/13_News_from_the_front.cfg

@@ -12,7 +12,7 @@
     turns=1
     snapshot=no
     next_scenario=14_Human_Alliance
-    theme=FullScreen
+    #theme=FullScreen
 
     {SCENARIO_MUSIC the_deep_path.ogg}