doc: revert 1e7d53bd so libpq chapter number is accessable

Fix PG 14 relnotes to use <link> instead of <xref>.  This was discussed
in commit message 59fa7eb6.

doc/src/sgml/libpq.sgml

 <!-- doc/src/sgml/libpq.sgml -->
 
-<chapter id="libpq" xreflabel="libpq">
+<chapter id="libpq">
  <title><application>libpq</application> &mdash; C Library</title>
 
  <indexterm zone="libpq">

doc/src/sgml/release-14.sgml

@@ -174,7 +174,7 @@ Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
 -->
 
 <para>
-Remove server and <xref linkend="libpq"/> support for the version 2 <link linkend="protocol">wire protocol</link> (Heikki Linnakangas)
+Remove server and <link linkend="libpq">libpq</link> support for the version 2 <link linkend="protocol">wire protocol</link> (Heikki Linnakangas)
 </para>
 
 <para>