Remove performance_bar JS from main.js

Currently we load the JS for the performance_bar through main.js which is not necessary as most users have most probably no performance bar so we should simply load in its own bundle and only if the performance bar is activated.

Main.JS Line: https://gitlab.com/gitlab-org/gitlab/blob/master/app/assets/javascripts/main.js#L166

How the CSS is loaded when activated: https://gitlab.com/gitlab-org/gitlab/blob/master/app/views/layouts/_head.html.haml#L57