Remove deprecated features in Airflow 2.0

[jlowin]

This issue is to track PRs that deprecated features and will require further action for Airflow 2.0. Even if those issues/PRs are closed, this issue should remain open as a reminder to remove the deprecations.

Issues include (list may be out of date -- best to search the code for deprecation warnings):
#1219 (utils refactor)
#1238/#1272 (standard imports)
#1285 (BaseOperator kwargs)
#1347 (unify CLI --yes arguments)