Resolve "Keyless signing with sigstore not working in child pipelines"

What does this MR do and why?

In the related issue, the infrastructure team are reporting failures integrating with Sigstore via cosign in child pipelines. grouppipeline security identified the root cause and pushed a fix in the original MR.

I've confirmed the fix is good and enabled the FF globally. This MR removes the FF as it is no longer needed.

References

• Issue: Keyless signing with sigstore not working in child pipelines
• Original MR: Resolve "Keyless signing with sigstore not working in child pipelines"
• FF: [FF] sigstore_child_pipelines_fix -- Derisk fix for sigstore signing in child pipelines
• Feature flag <code data-sourcepos="12:18-12:47">sigstore_child_pipelines_fix</code> has been set to <code data-sourcepos="12:65-12:71">true%</code> of actors on <strong data-sourcepos="12:86-12:93">gprd</strong>

Screenshots or screen recordings

https://gitlab.com/gitlab-org/gitlab/-/issues/422146#note_2832976172

MR acceptance checklist

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Related to #422146