Consider removing no-op background migration: LinkLfsObjects

The following discussion from !27558 (merged) should be addressed:

• @mayra-cabrera started a discussion:

  What are we going to do with Gitlab::BackgroundMigration::LinklfsObjects, are we going to remove that file at some point?

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Gitlab::BackgroundMigration::LinklfsObjects was reverted and made no-op since it was timing out. Let's consider removing the file.