Remove deprecated cli commands from the db group

[jedcunningham]

I've added these to the "legacy commands" list, so Airflow will return a
helpful error message if someone does run them.

e.g.

$ airflow db upgrade
Usage: airflow db [-h] COMMAND ...

Database operations

Positional Arguments:
  COMMAND
    check             Check if the database can be reached
    check-migrations  Check if migration have finished
    clean             Purge old records in metastore tables
    downgrade         Downgrade the schema of the metadata database.
    drop-archived     Drop archived tables created through the db clean command
    export-archived   Export archived data from the archive tables
    migrate           Migrates the metadata database to the latest version
    reset             Burn down and rebuild the metadata database
    shell             Runs a shell to access the database

Options:
  -h, --help          show this help message and exit

airflow db command error: argument COMMAND: Command `db upgrade` has been removed. Please use `airflow db migrate`, see help above.