doc: Remove dead link

The link to the NIST web page about DES standards leads to nowhere, and
according to archive.org has been forwarded to an unrelated page for
many years.  Therefore, just remove that link.  More up to date
information can be found via Wikipedia, for example.

doc/src/sgml/pgcrypto.sgml

@@ -1299,10 +1299,6 @@ gen_random_uuid() returns uuid
      </para>
      <para>Comparison of crypt-des, crypt-md5 and bcrypt algorithms.</para>
     </listitem>
-    <listitem>
-     <para><ulink url="http://csrc.nist.gov/cryptval/des.htm"></ulink></para>
-     <para>Standards for DES, 3DES and AES.</para>
-    </listitem>
     <listitem>
      <para>
       <ulink url="http://en.wikipedia.org/wiki/Fortuna_(PRNG)"></ulink>