remove debian entry from default steamos sources.list

vendor/steamos/sources.list.in

@@ -3,5 +3,3 @@
 
 deb http://repo.steampowered.com/steamos &steamos-codename; main contrib non-free
 deb-src http://repo.steampowered.com/steamos &steamos-codename; main contrib non-free
-
-deb http://cdn.debian.net/debian &stable-codename; main