Docs: remove codecov badge (#18623)

For a long time we haven't been using codecov, remove it so it wouldn't confuse
the readers

README.md

-# [Grafana](https://grafana.com) [![Circle CI](https://circleci.com/gh/grafana/grafana.svg?style=svg)](https://circleci.com/gh/grafana/grafana) [![Go Report Card](https://goreportcard.com/badge/github.com/grafana/grafana)](https://goreportcard.com/report/github.com/grafana/grafana) [![codecov](https://codecov.io/gh/grafana/grafana/branch/master/graph/badge.svg)](https://codecov.io/gh/grafana/grafana)
+# [Grafana](https://grafana.com) [![Circle CI](https://circleci.com/gh/grafana/grafana.svg?style=svg)](https://circleci.com/gh/grafana/grafana) [![Go Report Card](https://goreportcard.com/badge/github.com/grafana/grafana)](https://goreportcard.com/report/github.com/grafana/grafana) 
 
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