Alter required message using comment form api

Question

I prefer not to use asterisk markers on required fields in the comment form.

So I'd like to remove or alter the 'required fields are marked (*)' message.

Being a good wordpress user, I'd like to do it with my functions.php. I can work out how to filter most areas of the form, but not this. I'm guessing it will involve modifying the 'comment-notes-before' field of the 'defaults' array, but I can't quite see how to do that.