mysql: Remove unused job attributes

The columns were included to support job restarts. However, that has
never been implemented. So remove the unused columns.

Note: This change reduced the size of our mxq_job.ibd with 35824930 rows
from 32GB to 24GB which is more than I've expected.

mysql/create_tables.sql

@@ -102,10 +102,6 @@ CREATE TABLE IF NOT EXISTS mxq_job (
    date_start   TIMESTAMP NOT NULL DEFAULT 0,
    date_end     TIMESTAMP NOT NULL DEFAULT 0,
 
-   job_id_new     INT8 UNSIGNED   NULL DEFAULT NULL,
-   job_id_old     INT8 UNSIGNED   NULL DEFAULT NULL,
-   job_id_first   INT8 UNSIGNED   NULL DEFAULT NULL,
-
    stats_max_sumrss  INT8 UNSIGNED NOT NULL DEFAULT 0,
 
    stats_status   INT4 UNSIGNED NOT NULL DEFAULT 0,

mysql/migrate_014_remove_job_id_fields.sql

@@ -0,0 +1,4 @@
+ALTER TABLE mxq_job
+	DROP COLUMN job_id_new,
+	DROP COLUMN job_id_old,
+	DROP COLUMN job_id_first;