Doc: remove pg_prepared_statements.result_types from v15 docs.

This column is new in v16, but it was listed in the v15 docs too
via a back-patching fumble.

Per report from Peter Gigowski; diagnosis by Julien Rouhaud.

Discussion: https://postgr.es/m/CAM7cJ6XY_PAmx0kGn6U307EKZ+qXDFEBH27WP87-_ygetnBuxQ@mail.gmail.com

Author: tglsfdc

doc/src/sgml/system-views.sgml

@@ -1929,20 +1929,6 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
       </para></entry>
      </row>
 
-     <row>
-      <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>result_types</structfield> <type>regtype[]</type>
-      </para>
-      <para>
-       The types of the columns returned by the prepared statement in the
-       form of an array of <type>regtype</type>. The OID corresponding
-       to an element of this array can be obtained by casting the
-       <type>regtype</type> value to <type>oid</type>.
-       If the prepared statement does not provide a result (e.g., a DML
-       statement), then this field will be null.
-      </para></entry>
-     </row>
-
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>from_sql</structfield> <type>bool</type>