doc: Remove links to further reading from pgcrypto docs

The pgcrypto docs contained a set of links for useful reading and
technical references. These sets of links were however not actively
curated and had stale content and dead links. Rather than investing
time into maintining these, this removes them altogether since there
are lots of resources online which are actively maintained.

Backpatch to all supported versions since these links have been in
the docs for a long time.

Reported-by: Hanefi Onaldi <hanefi.onaldi@microsoft.com>
Reviewed-by: Magnus Hagander <magnus@hagander.net>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/170774255387.3279713.2822272755998870925@wrigleys.postgresql.org
Backpatch-through: v12

Author: danielgustafsson

doc/src/sgml/pgcrypto.sgml

@@ -1220,72 +1220,6 @@ gen_random_uuid() returns uuid
     ciphertexts of a given size.
    </para>
   </sect3>
-
-  <sect3 id="pgcrypto-notes-useful-reading">
-   <title>Useful Reading</title>
-
-   <itemizedlist>
-    <listitem>
-     <para><ulink url="https://www.gnupg.org/gph/en/manual.html"></ulink></para>
-     <para>The GNU Privacy Handbook.</para>
-    </listitem>
-    <listitem>
-     <para><ulink url="https://www.openwall.com/crypt/"></ulink></para>
-     <para>Describes the crypt-blowfish algorithm.</para>
-    </listitem>
-    <listitem>
-     <para>
-      <ulink url="https://www.iusmentis.com/security/passphrasefaq/"></ulink>
-     </para>
-     <para>How to choose a good password.</para>
-    </listitem>
-    <listitem>
-     <para><ulink url="http://world.std.com/~reinhold/diceware.html"></ulink></para>
-     <para>Interesting idea for picking passwords.</para>
-    </listitem>
-    <listitem>
-     <para>
-      <ulink url="http://www.interhack.net/people/cmcurtin/snake-oil-faq.html"></ulink>
-     </para>
-     <para>Describes good and bad cryptography.</para>
-    </listitem>
-   </itemizedlist>
-  </sect3>
-
-  <sect3 id="pgcrypto-notes-tech-ref">
-   <title>Technical References</title>
-
-   <itemizedlist>
-    <listitem>
-     <para><ulink url="https://tools.ietf.org/html/rfc4880"></ulink></para>
-     <para>OpenPGP message format.</para>
-    </listitem>
-    <listitem>
-     <para><ulink url="https://tools.ietf.org/html/rfc1321"></ulink></para>
-     <para>The MD5 Message-Digest Algorithm.</para>
-    </listitem>
-    <listitem>
-     <para><ulink url="https://tools.ietf.org/html/rfc2104"></ulink></para>
-     <para>HMAC: Keyed-Hashing for Message Authentication.</para>
-    </listitem>
-    <listitem>
-     <para>
-      <ulink url="https://www.usenix.org/legacy/events/usenix99/provos.html"></ulink>
-     </para>
-     <para>Comparison of crypt-des, crypt-md5 and bcrypt algorithms.</para>
-    </listitem>
-    <listitem>
-     <para>
-      <ulink url="https://en.wikipedia.org/wiki/Fortuna_(PRNG)"></ulink>
-     </para>
-     <para>Description of Fortuna CSPRNG.</para>
-    </listitem>
-    <listitem>
-     <para><ulink url="https://jlcooke.ca/random/"></ulink></para>
-     <para>Jean-Luc Cooke Fortuna-based <filename>/dev/random</filename> driver for Linux.</para>
-    </listitem>
-   </itemizedlist>
-  </sect3>
  </sect2>
 
  <sect2 id="pgcrypto-author">