Remove quality gate badge

I'm honest: a green badge looks good, but a red one doesn't and could
give a wrong impression. The gate usually fails because of duplicate
code reports - it's an indicator for improvement, but doesn't warrant
a "red flag".

Author: jeromegamez

README.md

@@ -9,7 +9,6 @@ Interact with [Google Firebase](https://firebase.google.com) from your PHP appli
 [![Discord](https://img.shields.io/discord/523866370778333184.svg?color=7289da&logo=discord)](https://discord.gg/nbgVfty)
 
 [![Build Status](https://travis-ci.org/kreait/firebase-php.svg?branch=master)](https://travis-ci.org/kreait/firebase-php)
-[![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=kreait_firebase-php&metric=alert_status)](https://sonarcloud.io/dashboard?id=kreait_firebase-php)
 [![Maintainability Rating](https://sonarcloud.io/api/project_badges/measure?project=kreait_firebase-php&metric=sqale_rating)](https://sonarcloud.io/dashboard?id=kreait_firebase-php)
 [![Reliability Rating](https://sonarcloud.io/api/project_badges/measure?project=kreait_firebase-php&metric=reliability_rating)](https://sonarcloud.io/dashboard?id=kreait_firebase-php)
 [![Coverage](https://sonarcloud.io/api/project_badges/measure?project=kreait_firebase-php&metric=coverage)](https://sonarcloud.io/dashboard?id=kreait_firebase-php)