Refactor code

- updated dao_get_letter_jobs_by_status to dao_get_letter_job_ids_by_status to return array of strings

app/celery/scheduled_tasks.py

@@ -13,7 +13,7 @@ from app.dao.date_util import get_month_start_and_end_date_in_utc
 from app.dao.inbound_sms_dao import delete_inbound_sms_created_more_than_a_week_ago
 from app.dao.invited_user_dao import delete_invitations_created_more_than_two_days_ago
 from app.dao.jobs_dao import (
-    dao_get_letter_jobs_by_status,
+    dao_get_letter_job_ids_by_status,
     dao_set_scheduled_jobs_to_pending,
     dao_get_jobs_older_than_limited_by
 )
@@ -308,7 +308,7 @@ def populate_monthly_billing():
 @notify_celery.task(name="run-letter-jobs")
 @statsd(namespace="tasks")
 def run_letter_jobs():
-    job_ids = [job.id for job in dao_get_letter_jobs_by_status(JOB_STATUS_READY_TO_SEND)]
+    job_ids = dao_get_letter_job_ids_by_status(JOB_STATUS_READY_TO_SEND)
     notify_celery.send_task(
         name=TaskNames.DVLA_FILES,
         args=(job_ids,),