reCaptcha display options are no longer supported

The display options for recaptcha are no longer supported in the way implemented by this extension and core.

They were used by recaptcha v1, which is no longer supported, see https://developers.google.com/recaptcha/old/docs/customization

I suggest we remove the setting from this extension (and I'll do an issue against core too when I get the chance).

There is a way to change the theme of the v2 checkbox between 'dark' and 'light', but I don't propose implementing that until core has as well, and it seems like an edge-case.