Delete gitter chat from Readme (#6282)

delete gitter chat from Readme, Slack will be enough

README.md

@@ -8,7 +8,7 @@ Dolphin Scheduler Official Website
 [![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=apache-dolphinscheduler&metric=alert_status)](https://sonarcloud.io/dashboard?id=apache-dolphinscheduler)
 [![Twitter Follow](https://img.shields.io/twitter/follow/dolphinschedule.svg?style=social&label=Follow)](https://twitter.com/dolphinschedule)
 [![Slack Status](https://img.shields.io/badge/slack-join_chat-white.svg?logo=slack&style=social)](https://join.slack.com/t/asf-dolphinscheduler/shared_invite/zt-omtdhuio-_JISsxYhiVsltmC5h38yfw)
-[![Join the chat at https://gitter.im/apache-dolphinscheduler/community](https://badges.gitter.im/apache-dolphinscheduler/community.svg)](https://gitter.im/apache-dolphinscheduler/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+