remove jobs from letter api calls

we now no longer create a job. At the end of the post there is no
action, as we don't have any tasks to queue immediately - if it's a
real notification it'll get picked up in the evening scheduled task.

If it's a test notification, we create it with an initial status of
sending so that we can be sure it'll never get picked up - and then we
trigger the update-letter-notifications-to-sent-to-dvla task to sent
the sent-at/by.

tests/app/notifications/test_process_letter_notifications.py

@@ -5,7 +5,6 @@ from app.models import Job
 from app.models import JOB_STATUS_READY_TO_SEND
 from app.models import LETTER_TYPE
 from app.models import Notification
-from app.notifications.process_letter_notifications import create_letter_api_job
 from app.notifications.process_letter_notifications import create_letter_notification
 from app.v2.errors import InvalidRequest
 from app.variables import LETTER_API_FILENAME