Remove uses of default.gui_bg/bg_img/slots

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

mods/bones/init.lua

@@ -13,9 +13,6 @@ end
 
 local bones_formspec =
 	"size[8,9]" ..
-	default.gui_bg ..
-	default.gui_bg_img ..
-	default.gui_slots ..
 	"list[current_name;main;0,0.3;8,4;]" ..
 	"list[current_player;main;0,4.85;8,1;]" ..
 	"list[current_player;main;0,6.08;8,3;8]" ..