Remove header noise from test_decoding test

Use psql's expanded output to avoid a pointless header.

Kyotaro Horiguchi, after an idea of Michael Paquier

Discussion: https://postgr.es/m/20181120050744.GJ4400@paquier.xyz

contrib/test_decoding/expected/ddl.out

@@ -723,8 +723,8 @@ SELECT pg_drop_replication_slot('regression_slot');
 (1 row)
 
 /* check that the slot is gone */
+\x
 SELECT * FROM pg_replication_slots;
- slot_name | plugin | slot_type | datoid | database | temporary | active | active_pid | xmin | catalog_xmin | restart_lsn | confirmed_flush_lsn 
------------+--------+-----------+--------+----------+-----------+--------+------------+------+--------------+-------------+---------------------
 (0 rows)
 
+\x

contrib/test_decoding/sql/ddl.sql

@@ -387,4 +387,6 @@ SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'inc
 SELECT pg_drop_replication_slot('regression_slot');
 
 /* check that the slot is gone */
+\x
 SELECT * FROM pg_replication_slots;
+\x