How can I remove the QR code button?

The simplest way to do that is to create some CSS file with the following content:

custom.css

[data-component="action.qr-code-button"] { display: none; }

Now upload that CSS file to your shop server in a directory which is publicly available. For Shopware this is usually the directory shopware-installation-path/public/ so the file path would be:

shopware-installation-path/public/custom.css

Now get the URL to the CSS file, for instance https://your-shop.de/custom.css and add this CSS file to your product designer configuration (Advanced Tab).

Read more about what you can do with this kind of customizations here: Monkey Patching