the only way to remove that warning is to disable the Quantity Buttons option in Customizer > Layout > Woocommerce.
Or just ignore it – the warning gets raised as some times a site uses Javascript to generate the URL the link will go to. This is not the case here as the <a> element is simply used to display a button that has no link.
It cannot be changed at this time.
I personally would ignore the warning – if the button was meant to link you to a page that you wanted crawled then i would agree its an error. But in this case its not a crawlable link as its not meant to link anywhere ๐