Remove unnecessary categories

[beatngu13]

The "What's changed" category is not necessary. The generated release notes already contain <h2>What's changed</h2>, the current configuration just adds an additional, nested <h3>What's changed</h3>:

https://github.com/gluonhq/gluonfx-maven-plugin/releases/tag/1.0.13

In contrast, here is the release.yml of a personal project without categories:

https://github.com/beatngu13/pdf-zoom-wizard/blob/develop/.github/release.yml

The generated release notes:

https://github.com/beatngu13/pdf-zoom-wizard/releases/tag/v0.11.0