patches: use a single patch for SKY4 instead of multiple

Some mods (eg, crusades.wad) try to use SKY4 to (re)define other
textures, which doesn’t work in vanilla Doom, and the normal E4 sky
ends up creating a medusa effect.

While it is sad to see our light source go, reducing the effectiveness
of Double Impact’s lighting effects built into some of the maps, we
should prefer mod compatibility with Ultimate Doom.

Resolves #589

lumps/textures/textures.cfg

@@ -1845,11 +1845,8 @@ WFALL4          64      128
 
 ; These textures are only in Ultimate Doom
 
-SKY4            1024    128
+SKY4            256     128
-*   SKY4A       0       0
+*   SKY4        0       0
-*   SKY4A       256     0
-*   SKY4B       512     0
-*   SKY4A       768     0
 
 #endif