remove fluxbb (unmaintained, dead link)

- unmaintained since 2021 https://github.com/fluxbb/fluxbb/commits/master
- ref. https://github.com/awesome-selfhosted/awesome-selfhosted/issues/2266
- unaddressed security issues

README.md

@@ -532,7 +532,6 @@ Simple deployment of [E-mail](https://en.wikipedia.org/wiki/Email) servers, e.g.
 - [EpochTalk](https://github.com/epochtalk/epochtalk) - Next Generation Forum Software. `MIT` `Nodejs`
 - [Flarum](https://flarum.org) - Delightfully simple forums. Flarum is the next-generation forum software that makes online discussion fun again. ([Source Code](https://github.com/flarum/flarum)) `MIT` `PHP`
 - [FlaskBB](https://flaskbb.org/) - FlaskBB is forum software written in Python using the microframework Flask. You can easily create new topics, posts and send other users private messages. It also includes basic administration and moderation tools. ([Source Code](https://github.com/flaskbb/flaskbb)) `BSD-3-Clause` `Python`
-- [FluxBB](https://fluxbb.org/) - Fast, light, user-friendly forum software for your website. ([Source Code](https://github.com/fluxbb/fluxbb)) `GPL-2.0` `PHP`
 - [Friendica](https://friendi.ca/) - Social Communication Server. ([Source Code](https://github.com/friendica/friendica)) `AGPL-3.0` `PHP`
 - [Glosa](https://github.com/glosa/glosa-server) - Open source commentary system easy to integrate with static pages. You can import from Disqus. `GPL-3.0` `Java`
 - [GNU social](https://gnu.io/social/) - Social communication software for both public and private communications. ([Source Code](https://notabug.org/diogo/gnu-social)) `AGPL-3.0` `PHP`