Remove old left-menu code

After we fully roll out the new Vue left-menu, remove the old left-menu code and the vue-left-menu feature toggle,

• Old left-menu (only used for mobile)
  • https://gitlab.com/gitlab-org/gitter/webapp/tree/fc20892381b2c23a0937d19329ea35a11282541e/public/js/views/menu/old
• Previous left-menu
  • https://gitlab.com/gitlab-org/gitter/webapp/tree/fc20892381b2c23a0937d19329ea35a11282541e/public/js/views/menu/room
  • https://gitlab.com/gitlab-org/gitter/webapp/tree/fc20892381b2c23a0937d19329ea35a11282541e/public/less/components/menu
  • Collections
• Remove the vue-left-menu and left-menu feature toggle
  • Wait for 20.0.0 to ship to production