Remove mentions of Redash

Redash has been shut down, but unfortunately the equivalent dashboards haven't been fully ported over to Looker for public consumption. For now, we'll just have to remove this information.

Related links:

• gitlab-com/Product#5 (closed)
• https://gitlab.com/meltano/looker/issues/294
• https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/4742