Drop test user when done with it.

Commit d7f8d26d added a test case that created a user, but forgot
to drop it again.  This is no good; for one thing, it causes repeated
"make installcheck" runs to fail.

src/test/regress/expected/stats_ext.out

@@ -756,3 +756,4 @@ DROP FUNCTION op_leak(int, int);
 RESET SESSION AUTHORIZATION;
 DROP VIEW priv_test_view;
 DROP TABLE priv_test_tbl;
+DROP USER regress_stats_user1;

src/test/regress/sql/stats_ext.sql

@@ -506,3 +506,4 @@ DROP FUNCTION op_leak(int, int);
 RESET SESSION AUTHORIZATION;
 DROP VIEW priv_test_view;
 DROP TABLE priv_test_tbl;
+DROP USER regress_stats_user1;