doc: remove useless MERGE example

Reported-by: dwayne.towell@gmail.com

Discussion: https://postgr.es/m/167699245721.1902146.6479762301617101634@wrigleys.postgresql.org

Backpatch-through: master

doc/src/sgml/ref/merge.sgml

@@ -707,23 +707,6 @@ WHEN NOT MATCHED THEN
 </programlisting>
   </para>
 
-  <para>
-   Notice that this would be exactly equivalent to the following
-   statement because the <literal>MATCHED</literal> result does not change
-   during execution.
-
-<programlisting>
-MERGE INTO customer_account ca
-USING (SELECT customer_id, transaction_value FROM recent_transactions) AS t
-ON t.customer_id = ca.customer_id
-WHEN MATCHED THEN
-  UPDATE SET balance = balance + transaction_value
-WHEN NOT MATCHED THEN
-  INSERT (customer_id, balance)
-  VALUES (t.customer_id, t.transaction_value);
-</programlisting>
-  </para>
-
   <para>
    Attempt to insert a new stock item along with the quantity of stock. If
    the item already exists, instead update the stock count of the existing