Remove keep latest artifact feature flags

What does this MR do?

Removes the keep_latest_artifact and destroy_only_unlocked_expired_artifacts feature flags.

Related #229936 (closed)

Does this MR meet the acceptance criteria?

Conformity

• N/A Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• N/A Separation of EE specific content