[grafana]- remove thanos historical datasource

What

Removes the "Thanos - Historical Data" datasource from grafana config.

Why

This won't actually remove the datasource.

The datasources unfortunately were defaulted to readonly when they were previously removed/renamed in !5384 (merged)

Because of this Grafana orphaned them in a readonly state and because the Thanos datasource wasn't editable, it created a new one.

Removing this will allow us to re-import the datasource after making the rename.