get_output_manager -> get_io_manager

Summary:
change get_output_manager to get_io_manager since its fetching the io_manager_key and expects an IOManager in a few call sites. Bumped in to this issue typing stuff in D6076

Just removed the output_manager decorator and tests since I think they don't make sense anymore, but let me know

Test Plan: bk

Reviewers: sandyryza, yuhan, cdecarolis

Reviewed By: sandyryza

Differential Revision: https://dagster.phacility.com/D6085