Remove broken github status badges from folly README

Reviewed By: LeeHowes

Differential Revision: D27302462

fbshipit-source-id: 8af6a8106563ded80de8ece803589fa07cc59241

README.md

 Folly: Facebook Open-source Library
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-[![linux](https://github.com/facebook/folly/workflows/linux/badge.svg)](https://github.com/facebook/folly/actions?query=workflow:linux)
-[![mac](https://github.com/facebook/folly/workflows/mac/badge.svg)](https://github.com/facebook/folly/actions?query=workflow:mac)
-[![windows](https://github.com/facebook/folly/workflows/windows/badge.svg)](https://github.com/facebook/folly/actions?query=workflow:windows)
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 ### What is `folly`?
 
 <img src="static/logo.svg" alt="Logo Folly" width="15%" align="right" />