remove outdated lgtm shield (prebid#9535)

Author: ChrisHuie

README.md

@@ -1,7 +1,6 @@
 [![Build Status](https://circleci.com/gh/prebid/Prebid.js.svg?style=svg)](https://circleci.com/gh/prebid/Prebid.js)
 [![Percentage of issues still open](http://isitmaintained.com/badge/open/prebid/Prebid.js.svg)](http://isitmaintained.com/project/prebid/Prebid.js "Percentage of issues still open")
 [![Coverage Status](https://coveralls.io/repos/github/prebid/Prebid.js/badge.svg)](https://coveralls.io/github/prebid/Prebid.js)
-[![Total Alerts](https://img.shields.io/lgtm/alerts/g/prebid/Prebid.js.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/prebid/Prebid.js/alerts/)
 
 # Prebid.js