Remove onboarding_step_full_uri feature flag

What does this MR do and why?

There have been no issues since this feature was enabled in production on Feb 25th and we are ready to remove the onboarding_step_full_uri feature flag.

References

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MR acceptance checklist

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Screenshots or screen recordings

No visual changes

How to set up and validate locally

1. Create a new user who will then enter onboarding.

2. In rails console set the new users step url to contain the full url.

   user = User.find_by(email: "replace-with-new-user-email@gitlab.com")
   user.onboarding_status_step_url = "http://127.0.0.1:3000/users/sign_up/groups/new"
   user.save!

3. Visit http://127.0.0.1:3000/groups.

4. Redirect loop should not happen.

Related to #520497 (closed)