grafana: Remove some duplicate dashboards

[f9n]

Maybe I'm missing something. I think some dashboards are doing similar things. Should we consider removing some of them? Or maybe we could merge a few of them? While merging, we could use Grafana rows to group and optionally hide some panels—so they can be expanded when needed. For example, we could add things like checkpointer and bgwriter to the DB Overview dashboard.

• postgres/v11/stat-activity-realtime.json vs postgres/v11/stat-activity.json
• postgres/v11/stat-statements-top-fast.json vs postgres/v11/stat-statements-top.json vs visual

[pashagolub]

Realtime means real-time data, like every 5 secs. Thus different storage. I'll keep it for a while. I need to rethink the whole concept first. Thanks for noticing!