Doc: remove long-obsolete advice about generated constraint names.

It's been twenty years since we generated constraint names that
look like "$N".  So this advice about double-quoting such names
is well past its sell-by date, and now it merely seems confusing.

Reported-by: Yaroslav Saburov <y.saburov@gmail.com>
Author: "David G. Johnston" <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/174393459040.678.17810152410419444783@wrigleys.postgresql.org
Backpatch-through: 13

doc/src/sgml/ddl.sgml

@@ -1470,9 +1470,6 @@ ALTER TABLE products ALTER COLUMN product_no SET NOT NULL;
 <programlisting>
 ALTER TABLE products DROP CONSTRAINT some_name;
 </programlisting>
-    (If you are dealing with a generated constraint name like <literal>$2</literal>,
-    don't forget that you'll need to double-quote it to make it a valid
-    identifier.)
    </para>
 
    <para>