Remove orphaned "unused repo" server code

Remove orphaned "unused repo" server code

We removed the "unused org" code in https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1715

Code first introduced in

• Backend: https://github.com/troupe/gitter-webapp/pull/1874
  • One of the commits on GitLab for reference: https://gitlab.com/gitlab-org/gitter/webapp/commit/d9b6e3bbf9364b34daa858b03656d5f8dc2fdbbf
• Frontend: https://github.com/troupe/gitter-webapp/pull/1914(frontend)
  • One of the commits on GitLab for reference: https://gitlab.com/gitlab-org/gitter/webapp/commit/42c1c6433d4119cab43c514b4acf2d48831620a3

Code usage removed in

• Frontend: https://github.com/troupe/gitter-webapp/pull/2068
  • One of the commits on GitLab for reference, https://gitlab.com/gitlab-org/gitter/webapp/commit/516ff0eb76f339c4349d6c35c6e619f0f296a073
• Backend: Removed in this commit/MR