Can I delete the footer reference?

Yes, you can. First create and activate a child theme of Terrifico.

https://codex.wordpress.org/Child_Themes

Then in the child theme directory that you created (name it /terrifico-child/) copy the original theme’s copyright.php into the /terrifico-child/ directory. Then modify that copy (not the original one) as you want.