Remove a mention of the Discord server

Author: bbatsov

README.md

@@ -1,7 +1,6 @@
 [![License GPL 3][badge-license]](http://www.gnu.org/licenses/gpl-3.0.txt)
 [![CI](https://github.com/bbatsov/prelude/workflows/CI/badge.svg)](https://github.com/bbatsov/prelude/actions/workflows/ci.yml)
 [![Patreon](https://img.shields.io/badge/patreon-donate-orange.svg)](https://www.patreon.com/bbatsov)
-[![Discord](https://img.shields.io/badge/chat-on%20discord-7289da.svg?sanitize=true)](https://discord.gg/3Cf2Qpyry5)
 
 Emacs Prelude
 =============