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/

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Once the date approaches closer, create a MR to remove it completely