FIX: remove migrate_from_s3 task that silently corrupts data (PR #11703)

Transient errors in migration are ignored, silently corrupting data, and the migration is incomplete and misses many sources of uploads, which will lead to an incorrect expectation of independence from the remote object storage after announcing that the migration was successful, regardles of whether transient errors permanently corrupted the data.

Remove this migration until such time as it is re-written to follow the same pattern has the migration to s3, moving the core logic out of the task.

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