Remove incorrect Coveralls badge. (#26550)

The code coverage for the repository seems to be ~85% while the coverage reported by the Coveralls badge is 100%.

README.md

 # <img src="https://flutter.io/images/flutter-mark-square-100.png" alt="Flutter" width="40" height="40" /> Flutter [![Join Gitter Chat Channel -](https://badges.gitter.im/flutter/flutter.svg)](https://gitter.im/flutter/flutter?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Build Status - Cirrus](https://api.cirrus-ci.com/github/flutter/flutter.svg)](https://cirrus-ci.com/github/flutter/flutter/master)
-[![Coverage Status -](https://coveralls.io/repos/github/flutter/flutter/badge.svg?branch=master)](https://coveralls.io/github/flutter/flutter?branch=master)
 
 
 # Build beautiful native apps in record time