remove unchanged header from changelog (#140)

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')