Do I need a child theme if I'm commenting lines out of functions.php?

I am using a theme which has a framework and a child theme already. To customise the site the way I want it, I’ve commented some lines out of functions.php. Of course the last time I updated the theme, the changes to functions.php were lost.

I have read that I can create a child theme for when I want to ADD to functions.php. What do I do when I want to REMOVE stuff?! Thanks.