Remove chatgpt_oauth_sign_in + link_omniauth_to_existing_user_on_login feature flags

Lee Tickettrequested to merge
gitlab-community/gitlab-org/gitlab:606823-cleanup-oai-ffs into master

What does this MR do and why?

Removes chatgpt_oauth_sign_in + link_omniauth_to_existing_user_on_login feature flags

Changelog: other

References

Screenshots or screen recordings

Before After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #606823

Merge request reports