You shouldn’t be stripping this via the functions.php file but rather edit the comments.php to remove the span directly.
If this is a commercial theme that receives updates then you should do it by creating a child theme and customizing your functions.php & comments.php in that child theme.
This is a great tutorial on how to do that: https://www.smashingmagazine.com/2016/01/create-customize-wordpress-child-theme/