Remove border radius setting from the Gutenberg button block?

Create a theme.json file in the root of your theme, and add this inside:

{
  "version": 1,
  "settings": {
    "blocks": {
      "core/button": {
        "border": {
          "customRadius": false
        }
      }
    }
  }
}

It will only work from WordPress v5.8 and above, because it requires Gutenberg v10. However Gutenberg v10 can be installed as a plugin on older versions of WordPress too, in which case it works with older WordPress versions as well, such as the currently latest stable release v5.7.2 (which is based on Gutenberg v9 by default).