Remove profiling queries from the dashboard component

[begelundmuller]

I have noticed that the dashboard component on ui.rilldata.com makes profiling queries. This is a problem because security policies are not well enforced for profiling queries, and we will soon remove access to them in the cloud.

I see two refactors that are needed, but there may be more:

• ColumnTimeRange is called for dashboards with a time series. It can be replaced with a call to MetricsViewTimeRange.
• TableColumns is called for dashboards that do not have a time series. Ideally this should be avoided altogether, but if needed, we can introduce a new safe API for getting the schema of a table.