Release FF link_fast_forward_merge_requests_to_deployment

What does this MR do and why?

This fully releases the link_fast_forward_merge_requests_to_deployment Feature Flag introduced in Link fast-forward MRs to deployment (!145211 - merged). Following the recommendation in the guide for a gitlab_com_derisk flag, we are removing/releasing the feature flag completely without setting it to true first.

This has already been fully enabled in gprd since 16 April 2024, see: #442377 (comment 1863662672)

MR acceptance checklist

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

Screenshots or screen recordings

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How to set up and validate locally

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Related to #442377 (closed)