Doc: remove 66c0185a3 from release notes.

Discussion: https://postgr.es/m/Zktzf926vslR35Fv@depesz.com
(also some private discussion among pgsql-release)

doc/src/sgml/release-17.sgml

@@ -404,17 +404,6 @@ This can be disabled using server variable enable_group_by_reordering.
 </para>
 </listitem>
 
-<!--
-Author: David Rowley <drowley@postgresql.org>
-2024-03-25 [66c0185a3] Allow planner to use Merge Append to efficiently impleme
--->
-
-<listitem>
-<para>
-Allow UNION (without ALL) to use MergeAppend (David Rowley)
-</para>
-</listitem>
-
 <!--
 Author: David Rowley <drowley@postgresql.org>
 2024-02-01 [9d1a5354f] Fix costing bug in MergeAppend