docs: remove Gitter badge from README file

- Remove the Gitter chat badge from the README file.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

README.md

@@ -4,7 +4,6 @@
 [![codecov](https://codecov.io/gh/gin-contrib/gzip/branch/master/graph/badge.svg)](https://codecov.io/gh/gin-contrib/gzip)
 [![Go Report Card](https://goreportcard.com/badge/github.com/gin-contrib/gzip)](https://goreportcard.com/report/github.com/gin-contrib/gzip)
 [![GoDoc](https://godoc.org/github.com/gin-contrib/gzip?status.svg)](https://godoc.org/github.com/gin-contrib/gzip)
-[![Join the chat at https://gitter.im/gin-gonic/gin](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/gin-gonic/gin)
 
 Gin middleware to enable `GZIP` support.