CLN: remove _from_mgr (no longer used after GH-45363)#45415

Merged

jreback merged 2 commits intopandas-dev:mainfrom

jorisvandenbossche:groupby-subclass-follow-up

Jan 19, 2022

Commits on Jan 17, 2022

CLN: move __finalize__ call into splitter (groupbt GH-45363 follow-up)
jorisvandenbossche
committed

Commits on Jan 18, 2022

Merge remote-tracking branch 'upstream/main' into groupby-subclass-follow-up
jorisvandenbossche
committed