This plugin has been removed from the Jenkins Update Center
The Folders plugin was made Open Source. The other plugins rely on the RUN@cloud service that CloudBees has shut down. As a result there is no requirement for this plugin to remain in existence.
To uninstall, simply disable or delete the CloudBees Free Enterprise Plugins plugin and an any of the proprietary plugins that you want. If you disable only the CloudBees Free Enterprise Plugins you will no longer be able to access the CloudBees proprietary update center. If you disable or delete only the CloudBees proprietary plugins, they will not be reinstalled unless you upgrade the CloudBees Free Enterprise Plugins plugin to a newer version.