Deprecate influxdb

Announced https://about.gitlab.com/blog/2019/12/22/gitlab-12-6-released/#gitlab-13.0-will-drop-support-for-influxdb-self-monitoring-metrics

Remove from Docs https://gitlab.com/gitlab-org/grafana-dashboards/tree/master

Implementation

My plan is to break this into 4 MRs:

1. Views/Docs: !30786 (merged)
2. Entry points: !31149 (merged)
3. Supporting libs: !31404 (merged)
4. Database columns: Discuss if we should in #217850
5. GDK: gitlab-development-kit#927 (closed)