Remove letter-jobs api

When we first built letters you could only send them via a CSV upload, initially we needed a way to send those files to dvla per job.
We since stopped using this page. So let's delete it!

app/dao/jobs_dao.py

@@ -144,21 +144,3 @@ def dao_get_all_letter_jobs():
     ).order_by(
         desc(Job.created_at)
     ).all()
-
-
-def dao_get_letter_job_ids_by_status(status):
-    jobs = db.session.query(
-        Job
-    ).join(
-        Job.template
-    ).filter(
-        Job.job_status == status,
-        Template.template_type == LETTER_TYPE,
-        # test letter jobs (or from research mode services) are created with a different filename,
-        # exclude them so we don't see them on the send to CSV
-        Job.original_file_name != LETTER_TEST_API_FILENAME
-    ).order_by(
-        desc(Job.created_at)
-    ).all()
-
-    return [str(job.id) for job in jobs]