aslpavel and facebook-github-bot remove BlobRepo::rechunk_file_by_content_id
Summary: `BlobRepo::rechunk_file_by_content_id` only used once and just calls `filestore::rechunk`, so it is safe to remove it

Reviewed By: StanislavGlebik

Differential Revision: D19691267

fbshipit-source-id: f40b4dc14295f91f6134c29ddbfeaa8abd66535e
Latest commit b451af2 Feb 3, 2020