README: Remove obsolete badges

We use github actions for CI now. The travis and appveyor badges are
obsolete.

README.md

@@ -1,8 +1,6 @@
 # QGroundControl Ground Control Station
 
 [![Releases](https://img.shields.io/github/release/mavlink/QGroundControl.svg)](https://github.com/mavlink/QGroundControl/releases)
-[![Travis Build Status](https://travis-ci.org/mavlink/qgroundcontrol.svg?branch=master)](https://travis-ci.org/mavlink/qgroundcontrol)
-[![Appveyor Build Status](https://ci.appveyor.com/api/projects/status/crxcm4qayejuvh6c/branch/master?svg=true)](https://ci.appveyor.com/project/mavlink/qgroundcontrol)
 
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/mavlink/qgroundcontrol?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)