We do not use "AUTHORS" anymore. At #7319, the whole handling was changed. It seems that there was forgotton to delete the AUTHORS file itself. This PR fixes that.

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.