test_decoding: Remove unnecessary table in twophase test

The end of this test is dropping all the relations created but forgot
about this one.  This is not critical, but let's be clean, and the test
expects a cleanup, as documented.

Author: Nishant Sharma
Discussion: https://postgr.es/m/CADrsxdb0ueGV9nrC6s8zvXLkGUhnEjx7Ou_p5wo38TvmSvF83A@mail.gmail.com

contrib/test_decoding/expected/twophase.out

@@ -209,6 +209,7 @@ SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'inc
 -- cleanup and make sure results are also empty
 DROP TABLE test_prepared1;
 DROP TABLE test_prepared2;
+DROP TABLE test_prepared_savepoint;
 -- show results. There should be nothing to show
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
  data 

contrib/test_decoding/sql/twophase.sql

@@ -108,6 +108,7 @@ SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'inc
 -- cleanup and make sure results are also empty
 DROP TABLE test_prepared1;
 DROP TABLE test_prepared2;
+DROP TABLE test_prepared_savepoint;
 -- show results. There should be nothing to show
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');