As developer of this product I will not call on any legal action or go on a manhunt if you remove the attribution link(not that I don't want to :D ). so if you truly need to remove the attribution and can't pay 20$ to have the rights, just do it and don't feel bad about it.
although it would still be more ethical if you could add our link to your website's "About" page as a way of saying thank you.
to remove or change the footer link edit "inc/blocks.php" in your theme directory and search for our attribution link to replace or remove it. it is currently located in line 867.