Discord is the official channel - update README (#3943)

Matrix (and IRC) bridge drop too many messages which frustrate new user.
Let's only promote the official communication tool.

README.md

@@ -4,7 +4,6 @@
 
 [![Build Status](https://drone.gitea.io/api/badges/go-gitea/gitea/status.svg)](https://drone.gitea.io/go-gitea/gitea)
 [![Join the Discord chat at https://discord.gg/NsatcWJ](https://img.shields.io/discord/322538954119184384.svg)](https://discord.gg/NsatcWJ)
-[![Join the Matrix chat at https://matrix.to/#/#gitea:matrix.org](https://img.shields.io/badge/matrix-%23gitea%3Amatrix.org-7bc9a4.svg)](https://matrix.to/#/#gitea:matrix.org)
 [![](https://images.microbadger.com/badges/image/gitea/gitea.svg)](https://microbadger.com/images/gitea/gitea "Get your own image badge on microbadger.com")
 [![codecov](https://codecov.io/gh/go-gitea/gitea/branch/master/graph/badge.svg)](https://codecov.io/gh/go-gitea/gitea)
 [![Go Report Card](https://goreportcard.com/badge/code.gitea.io/gitea)](https://goreportcard.com/report/code.gitea.io/gitea)