Cleanup update_oauth_registration_flow feature flag

What does this MR do and why?

The update_oauth_registration_flag which redirects single-sign on users to the onboarding flow has been rolled out to 100% of the users for 3 months now (https://gitlab.com/gitlab-org/gitlab/-/issues/364342#note_1104728994). We haven't noticed any issues so far, so this MR just cleans up the feature flag and keeps the updated code.

Screenshots or screen recordings

Screen_Recording_2023-01-02_at_9.30.20_pm

How to set up and validate locally

1. Turn off admin approval for sign-up

ApplicationSetting.first.update(require_admin_approval_after_user_signup: false)

1. Configure Github as an OAuth provider in config/gitlab.yml file

  omniauth:
    allow_single_sign_on: true
    block_auto_created_users: false
    providers:
    - { name: 'github',                                                         
        app_id: 'YOUR_APP_ID',
        app_secret: 'YOUR_APP_SECRET',
        verify_ssl: false,
        provider_ignores_state: true,
        args: { scope: 'user:email' } }

1. Register a new user on http://localhost:3000/users/sign_up

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #371551 (closed)