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.