What are "nonremovable row versions"?
They are your data. You probably don't want to remove them ;)

"nonremovable row versions" includes the current, active row versions containing the data you care about. It also includes row versions made obsolete by a delete or update, but still visible to the snapshot of one or more running transactions.

You should only be concerned if there are lots of nonremovable dead row versions. That is not the case here. To learn more, read the manual on routine vacuuming, the chapter on concurrency and MVCC, and read about table bloat.