doc: remove pg_config mention from PG 12 release notes

Reported-by: Tom Lane

Discussion: https://postgr.es/m/28209.1556556696@sss.pgh.pa.us

doc/src/sgml/release-12.sgml

@@ -2012,17 +2012,6 @@ Allow the <literal>XML BY VALUE</literal> clause (Chapman Flack)
 <para>
 The clause has no effect.
 </para>
-</listitem>
-
-<listitem>
-<!--
-Author: Joe Conway <mail@joeconway.com>
-2019-02-17 [290e3b77f] Mark pg_config() stable rather than immutable
--->
-
-<para>
-Mark <function>pg_config()</function> stable rather than immutable (Joe Conway)
-</para>
 </listitem>
 
     </itemizedlist>