Remove :track_mr_picking feature flag - Track merge requests of cherry-picked commits when deploying

With !22209 (merged) we developed #36130 (closed) making use of a feature flag.

Once the feature has been validated on all the environments, we need to remove track_mr_picking feature flag.

Once the feature flag is removed, it will be available for users

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.