[AIRFLOW-XXX] Remove default/wrong values from test config. (#5684)
[AIRFLOW-XXX] Remove default/wrong values from test config. (#5684)

Most of the values I've removed here are the current defaults, so we
don't need to specify them again.

The reason I am removing them is that `email_backend` of
`airflow.utils.send_email_smtp` has been incorrect since 1.7.2(!) but
hasn't mattered until #5379 somehow triggered it. By removing the
default values it should make it easier to update in future.

(cherry picked from commit b5b9bc1e79b1bd0a592ac7269fd0c8366758fa81)

owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow