Remove old left-menu code

Fix https://gitlab.com/gitlab-org/gitter/webapp/issues/2195

• Remove old left-menu code
• Remove usage of vue-left-menu and always use new-world
• Remove frame-utils

Tools

• webpack-unused
  • npm run webpack -- --config public/js/webpack.config.js --json | webpack-unused -s public
• depcheck
  • depcheck
• trucker
  • trucker --unused

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Other MRs where we cleaned up stuff

• https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1552
• https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1289

Testing strategy

Because of the amount of deletions, this isn't really viable to review completely

• Startup the app and see that it still works

TODO

• Look at mobile(Android/iOS) asset builds