Dead 3rd party app: Developers Talk for Android

The Developers Talk for Android 3rd party app links to a 404 and should be removed.

The relevant code is here if you want to create a merge request, https://gitlab.com/gitlab-org/gitter/webapp/blob/715f47451b2541281c9e056df4dc14c9852a60fe/public/templates/apps.hbs#L65