Make WALLIFIED_MAZE into a level flag.

It should now be randomly disabled for a 3rd of Gehennom, to make things
a tad more interesting there. It's also disabled in Baalzebub's lair,
to make things a little more interesting.

Still don't know why the beetle is disappearing.

util/makedefs.c

@@ -1367,9 +1367,6 @@ static const char *build_opts[] = {
 #ifdef VISION_TABLES
 		"vision tables",
 #endif
-#ifdef WALLIFIED_MAZE
-		"walled mazes",
-#endif
 #ifdef ZEROCOMP
 		"zero-compressed save files",
 #endif