doc: drop Prow build badge

Signed-off-by: Stefan Büringer [email protected]

Author: sbueringer

README.md

@@ -1,7 +1,5 @@
 <a href="https://cluster-api.sigs.k8s.io"><img alt="capi" src="./docs/book/src/images/introduction.png" width="160x" /></a>
 <p>
-<a href="https://prow.k8s.io/?job=post-cluster-api-push-images">
-<img alt="Build Status" src="https://prow.k8s.io/badge.svg?jobs=post-cluster-api-push-images"></a>
 <a href="https://godoc.org/sigs.k8s.io/cluster-api"><img src="https://godoc.org/sigs.k8s.io/cluster-api?status.svg"></a>
 <!-- join kubernetes slack channel for cluster-api -->
 <a href="http://slack.k8s.io/">