Doc: remove duplicated step in RLS example.

Seems to have been a copy-and-paste mistake in 093129c9.
Per report from max1@inbox.ru.

Discussion: https://postgr.es/m/161591740692.24273.4202054598867879464@wrigleys.postgresql.org

doc/src/sgml/ddl.sgml

@@ -2495,12 +2495,6 @@ CREATE POLICY admin_local_only ON passwd AS RESTRICTIVE TO admin
  127.0.0.1
 (1 row)
 
-=> SELECT current_user;
- current_user 
---------------
- admin
-(1 row)
-
 => TABLE passwd;
  user_name | pwhash | uid | gid | real_name | home_phone | extra_info | home_dir | shell
 -----------+--------+-----+-----+-----------+------------+------------+----------+-------