remove unchanged header from changelog

It's always been confusing as it makes it look like everything below is
"unreleased". This is because we never update this section and so it
looks like a header over the rest of the doc.

CHANGELOG.md

@@ -5,8 +5,6 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
-
 ## [6.0.4] - 2024-07-25
 ### Changed
 - Fixed issue related to the method 'GetObjects()' (specifically related to the attribute 'Classification')