[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.