Remove build status badge from README (#3195)

## Issue Addressed

Removes the build status badge from the main README.md. I don't think it actually serves a purpose and it also has the downside that a spurious failure gives us a red badge. For example, v2.2.1 failed with a [spurious failure](https://github.com/sigp/lighthouse/runs/5984392665?check_suite_focus=true) and I can't see a way to re-trigger that run. It will be red until our next release.

The same test suite runs when we merge into `unstable`, so those tests must have already passed in order for the commits to get onto `stable` (assuming our workflow is followed). Github will send notifications on failed CI, so we'll still be alerted to a failure without checking this badge.

README.md

@@ -2,10 +2,8 @@
 
 An open-source Ethereum consensus client, written in Rust and maintained by Sigma Prime.
 
-[![Build Status]][Build Link] [![Book Status]][Book Link] [![Chat Badge]][Chat Link]
+[![Book Status]][Book Link] [![Chat Badge]][Chat Link]
 
-[Build Status]: https://github.com/sigp/lighthouse/workflows/test-suite/badge.svg?branch=stable
-[Build Link]: https://github.com/sigp/lighthouse/actions
 [Chat Badge]: https://img.shields.io/badge/chat-discord-%237289da
 [Chat Link]: https://discord.gg/cyAszAh
 [Book Status]:https://img.shields.io/badge/user--docs-unstable-informational