Remove uses of default.gui_bg/bg_img/slots

Keep their definitions, to not break mods, but move them to legacy.lua.

mods/vessels/init.lua

@@ -3,9 +3,6 @@
 
 local vessels_shelf_formspec =
 	"size[8,7;]" ..
-	default.gui_bg ..
-	default.gui_bg_img ..
-	default.gui_slots ..
 	"list[context;vessels;0,0.3;8,2;]" ..
 	"list[current_player;main;0,2.85;8,1;]" ..
 	"list[current_player;main;0,4.08;8,3;8]" ..