Remove Gitlab-Agentk-Api-Request header

What does this MR do and why?

Remove Gitlab-Agentk-Api-Request header. Rely on Gitlab-Agent-Api-Request header instead.

As part of Remove Gitlab-Agentk-Api-Request header (gitlab-org/cluster-integration/gitlab-agent!2697 - merged) , KAS no longer sends Gitlab-Agentk-Api-Request header. This MR cleans up the related code in Rails.

References

• Remove old authentication header for call made ... (gitlab-org/cluster-integration/gitlab-agent#711 - closed)

Screenshots or screen recordings

N.A.

How to set up and validate locally

1. Run GitLab.
2. Run KAS.
3. Run agentk.
4. Verify things are running as expected.

MR acceptance checklist

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