Remove unused roomService.createRoomByUri()

Remove unused code and endpoints related to roomService.createRoomByUri()

• Remove /api/private/create-github-room endpoint
  • Don't see any usage in the iOS, and Android app
  • It was used in the webapp confirm modal which was used with the non-Vue.js left-menu's (no longer used). Here are some past commits of usage:
    • https://gitlab.com/gitlab-org/gitter/webapp/blob/c157d7093b0fa6bf0a9716d57552a8ccbec05571/public/js/views/menu/room/base-collection/base-collection-view.js#L234
    • https://gitlab.com/gitlab-org/gitter/webapp/blob/c157d7093b0fa6bf0a9716d57552a8ccbec05571/public/js/views/menu/old/room-collection-view.js#L206
    • https://gitlab.com/gitlab-org/gitter/webapp/blob/c157d7093b0fa6bf0a9716d57552a8ccbec05571/public/js/views/menu/old/orgCollectionView.js#L58
• Remove room creation confirmation modal and clientside route #confirm/:uri
• Remove roomService.createRoomByUri
• Remove roomService.createRoomForGitHubUri

This MR started out as adding GitLab room creation support to roomService.createRoomByUri() but discovered the following the line all the way down shows it's no longer in use.

Done while working on https://gitlab.com/gitlab-org/gitter/webapp/issues/2393