Remove import_relation_object_persistence feature flag

What does this MR do and why?

This MR cleans up import_relation_object_persistence. It has been rolled out on .com for 4 months #354497 (comment 873925289)

I am not updating it to default_enabled: true first, because as per the rollout procedure this is optional, and since the feature flag is performance related and has been enabled for 4 months on .com, I am cleaning it up straight away.

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