Remove "grass under snow" from default:dirt_with_snow in darkness.

This is technically "dirt with grass" that's just under a snow
cover, so in darkness the grass on these nodes will also die,
turning it into dirt.

This doesn't convert dirt_with_snow under snow.

mods/default/functions.lua

@@ -384,7 +384,11 @@ minetest.register_abm({
 --
 
 minetest.register_abm({
-	nodenames = {"default:dirt_with_grass", "default:dirt_with_dry_grass"},
+	nodenames = {
+		"default:dirt_with_grass",
+		"default:dirt_with_dry_grass",
+		"default:dirt_with_snow",
+	},
 	interval = 8,
 	chance = 50,
 	catch_up = false,