Remove OpenAI workhorse and API key references

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/149727



Merged-by: Vitali Tatarintev <vtatarintev@gitlab.com>
Approved-by: Aishwarya Subramanian <asubramanian@gitlab.com>
Approved-by: Vitali Tatarintev <vtatarintev@gitlab.com>
Co-authored-by: Michael Thomas <micthomas@gitlab.com>