Clean up clone_job_variables_at_job_retry

What does this MR do and why?

Cleans up the clone_job_variables_at_job_retry feature flag that switches copying job variables when retrying job.

Original MR !75042 (merged)

Revert MR !75682 (merged)

Revert of the revert MR !75720 (merged)

Related incident gitlab-com/gl-infra/production#5991 (closed)

Feature Issue #327347 (closed)

Feature Flag Issue #347156 (closed)

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