README: remove Go Report Card badge

The project was sunset;

> After more than a decade of serving the ecosystem, the time
> has come to sunset Go Report Card. Following the loss of our
> primary infrastructure sponsor, maintaining the web app is
> no longer sustainable.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

README.md

@@ -2,7 +2,6 @@
 [![PkgGoDev](https://img.shields.io/badge/go.dev-docs-007d9c?style=flat-square&logo=go&logoColor=white)](https://pkg.go.dev/github.com/docker/docker-credential-helpers)
 [![Build Status](https://img.shields.io/github/actions/workflow/status/docker/docker-credential-helpers/build.yml?label=build&logo=github&style=flat-square)](https://github.com/docker/docker-credential-helpers/actions?query=workflow%3Abuild)
 [![Codecov](https://img.shields.io/codecov/c/github/docker/docker-credential-helpers?logo=codecov&style=flat-square)](https://codecov.io/gh/docker/docker-credential-helpers)
-[![Go Report Card](https://goreportcard.com/badge/github.com/docker/docker-credential-helpers?style=flat-square)](https://goreportcard.com/report/github.com/docker/docker-credential-helpers)
 
 ## Introduction