Remove a question that’s irrelevant for invitation

What does this MR do and why?

Closes #354799 (closed)

This removes a question from the saas registration flow when accepting an invitation. This streamlines that flow a little bit and backfills a couple tests that should be present around it.

Screenshots or screen recordings

Invitation registration flow:

Standard registration flow:

How to set up and validate locally

It's actually easiest to review the setup steps in the feature spec, because there are several feature flags that matter for the overall experience that reflects what's happening on gitlab.com production currently.

MR acceptance checklist

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