Remove deprecated report card badge (#78)

Author: bluekeyes

README.md

@@ -1,6 +1,6 @@
 # go-gitdiff
 
-[![PkgGoDev](https://pkg.go.dev/badge/github.com/bluekeyes/go-gitdiff/gitdiff)](https://pkg.go.dev/github.com/bluekeyes/go-gitdiff/gitdiff) [![Go Report Card](https://goreportcard.com/badge/github.com/bluekeyes/go-gitdiff)](https://goreportcard.com/report/github.com/bluekeyes/go-gitdiff)
+[![PkgGoDev](https://pkg.go.dev/badge/github.com/bluekeyes/go-gitdiff/gitdiff)](https://pkg.go.dev/github.com/bluekeyes/go-gitdiff/gitdiff)
 
 A Go library for parsing and applying patches generated by `git diff`, `git
 show`, and `git format-patch`. It can also parse and apply unified diffs