Remove incorrect warning from pg_archivecleanup document.

The .backup file name can be passed to pg_archivecleanup even if
it includes the extension which is specified in -x option.
However, previously the document incorrectly warned a user
not to do that.

Back-patch to 9.2 where pg_archivecleanup's -x option and
the warning were added.

doc/src/sgml/ref/pgarchivecleanup.sgml

@@ -130,11 +130,6 @@ pg_archivecleanup:  removing file "archive/00000001000000370000000E"
         .gz</literal>.
        </para>
 
-       <para>
-        Note that the
-        <filename>.backup</> file name passed to the program should not
-        include the extension.
-       </para>
       </listitem>
      </varlistentry>