I think that this button is redundant in Zen Cart, it's the equivalent of ZC's Confirm/Pay Now/Confirm Order button at the end of step 3. Due to the way the surrounding divs overlap you would only save 17px if you 'removed' it.
In browser dev tools try adding display:none; to the element and you'll see that that hides it fully. I guess you would need to change the 'visibility: hidden;' to 'display:none;' in includes\modules\pages\checkout_payment\jscript_square_webPay.php (try searching it for 'card-button').
For me, it would be more trouble than it's worth given I would be editing a plugins core file.
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