Remove Gitter Topics from the codebase
We deprecated topics on 2018-7-16 and plan to remove it completely from the UI on 2018-9-1
- https://gitlab.com/gitlab-org/gitter/webapp/issues/1946
- https://blog.gitter.im/2018/07/16/gitter-topics-deprecation-notice/
Once the date approaches closer, create a MR to remove it completely