Update README.md

Remove NGINX Community Slack, it's been sunsetted

Author: Sergey A. Osokin

README.md

@@ -1,7 +1,6 @@
 ![GitHub go.mod Go version](https://img.shields.io/github/go-mod/go-version/nginx/agent)
 ![GitHub License](https://img.shields.io/github/license/nginx/agent)
 ![Contributions Welcome](https://img.shields.io/badge/contributions-welcome-brightgreen.svg?style=flat)
-[![Slack](https://img.shields.io/badge/slack-join%20us-brightgreen.svg?logo=slack)](https://nginxcommunity.slack.com/channels/nginx-agent)
 ![coverage](https://raw.githubusercontent.com/nginx/agent/badges/.badges/v3/coverage.svg)
 
 # NGINX Agent