Record job success without deleting the job record

Previously, the records for jobs would be deleted. It's useful to know when
jobs were inserted in to the database, as well as when they succeeded (if they
have). This change also makes it possible to keep track of jobs that have
failed, as they won't be deleted.

sqitch/revert/dates_to_load_new_guix_revision_jobs.sql

@@ -0,0 +1,8 @@
+-- Revert guix-data-service:dates_to_load_new_guix_revision_jobs from pg
+
+BEGIN;
+
+ALTER TABLE load_new_guix_revision_jobs DROP COLUMN created_at;
+ALTER TABLE load_new_guix_revision_jobs DROP COLUMN succeeded_at;
+
+COMMIT;