Default/trees: Clean-up 'can grow' function

mods/default/trees.lua

@@ -16,9 +16,9 @@ local function can_grow(pos)
 	if is_soil == 0 then
 		return false
 	end
-	local ll = minetest.get_node_light(pos)
+	local light_level = minetest.get_node_light(pos)
-	if not ll or ll < 13 then -- Minimum light level for growth
+	if not light_level or light_level < 13 then
-		return false          -- matches grass, wheat and cotton
+		return false
 	end
 	return true
 end