Reintroduce lfs_link_existing_object feature flag

Igor Drozdovrequested to merge
id-reintroduce-lfs-link-existing-object-ff into master

What does this MR do?

We have an issue when mirrors don't download new LFS files due to this optimization

Let's reintroduce the feature flag until we figure out how to fix the issue

Related issue: #331364 (closed)

Edited by Igor Drozdov

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