doc: Remove cirrus-ci build badge.

README.md

@@ -6,7 +6,6 @@
 [![GoDoc](https://godoc.org/github.com/google/mtail?status.png)](http://godoc.org/github.com/google/mtail)
 [![Go Report Card](https://goreportcard.com/badge/github.com/google/mtail)](https://goreportcard.com/report/github.com/google/mtail)
 [![OSS-Fuzz](https://oss-fuzz-build-logs.storage.googleapis.com/badges/mtail.svg)](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:mtail)
-[![CirrusCI Build Status](https://api.cirrus-ci.com/github/google/mtail.svg)](https://cirrus-ci.com/github/google/mtail)
 [![codecov](https://codecov.io/gh/google/mtail/branch/main/graph/badge.svg)](https://codecov.io/gh/google/mtail)
 
 `mtail` is a tool for extracting metrics from application logs to be exported