Remove the demo section in the README

Heroku removed the free tier possibility and the demo does not work anymore.

README.md

@@ -39,17 +39,6 @@ Read more about Alchemy on the [website](https://alchemy-cms.com) and in the [gu
 - Flexible caching
 - Hostable on any Server that supports Ruby on Rails, a SQL Database and ImageMagick
 
-## 🎮 Demo
-
-Deploy your own free demo on Heroku
-
-[![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/AlchemyCMS/alchemy-demo)
-
-or visit the existing demo at https://alchemy-demo.herokuapp.com
-
-- Login: `demo`
-- Password: `demo123`
-
 ## 🚂 Rails Version
 
 **This version of AlchemyCMS runs with Rails 7.0**