Remove tagtool - it's no longer maintained

Last release of https://gitlab.com/tagtool/tagtool was more than year ago, also website doesn't work (it has been replaced by some splash screen). This is no longer awesome.

README.md

@@ -904,7 +904,6 @@ _Games, game servers and control panels._
 
 - [Mindmaps](https://github.com/drichard/mindmaps) - Open source, offline capable, mind mapping application. ([Demo](http://drichard.org/mindmaps/)) `AGPL-3.0` `HTML5`
 - [My Mind](https://github.com/ondras/my-mind) - Web application for creating and managing mind maps. ([Demo](https://my-mind.github.io/?url=examples%2Ffeatures.mymind)) `MIT` `Javascript`
-- [TagTool](https://letstag.it) - Collaborative knowledge management tool. Create word clouds and link the elements. ([Demo](https://demo.letstag.it), [Source Code](https://gitlab.com/tagtool/tagtool)) `MIT` `Python`
 - [Weaviate](https://github.com/semi-technologies/weaviate) - A cloud-native, realtime vector search engine integrating scalable machine learning models (GraphQL and RESTful API's). ([Demo](https://www.semi.technology/documentation/weaviate/current/)) `BSD-3-Clause` `Go`
 
 ## Learning and Courses