CLN: remove _from_mgr (no longer used after GH-45363)
One part of the follow-up to https://github.com/pandas-dev/pandas/pull/45363

This one doesn't necessarily need to be backported to 1.4
we just merged some cleanup similar to this by @jbrockmendel so if you can rebase @jbrockmendel it would be useful if you can mention the PR when doing a follow-up to a specific PR (so that github shows it as a linked PR in the original PR) thanks @jorisvandenbossche 