chore: remove unused badge

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

README.md

@@ -5,7 +5,6 @@
 [![codecov](https://codecov.io/gh/appleboy/drone-packer/branch/master/graph/badge.svg)](https://codecov.io/gh/appleboy/drone-packer)
 [![Go Report Card](https://goreportcard.com/badge/github.com/appleboy/drone-packer)](https://goreportcard.com/report/github.com/appleboy/drone-packer)
 [![Docker Pulls](https://img.shields.io/docker/pulls/appleboy/drone-packer.svg)](https://hub.docker.com/r/appleboy/drone-packer/)
-[![microbadger](https://images.microbadger.com/badges/image/appleboy/drone-packer.svg)](https://microbadger.com/images/appleboy/drone-packer "Get your own image badge on microbadger.com")
 [![Build status](https://ci.appveyor.com/api/projects/status/pmkfbnwtlf1fm45l/branch/master?svg=true)](https://ci.appveyor.com/project/appleboy/drone-packer/branch/master)
 
 Drone plugin for build Automated machine images with [Packer](https://www.packer.io/). For the usage information and a listing of the available options please take a look at [the docs](http://plugins.drone.io/appleboy/drone-packer/).