When you delete a link, it is permanently removed from your account and no longer works when clicked. If there's a QR Code associated with it, the QR Code no longer works, either. If someone scans the QR Code or clicks the link after you've deleted it, they're taken to a Bitly error page.
Link deletion is permanent — neither the link nor the QR Code be recovered after the link is deleted. Be sure to confirm your QR Code is not actively in use (for example, on printed materials) before deleting the link.