Remove "Stage" variable

as we will use distinct Influx and Grafana instances for the different stages/environments.

deploy/grafana-dashboard/queries/runner-startup-duration.flux

@@ -5,6 +5,5 @@ result = from(bucket: "poseidon")
   |> filter(fn: (r) => r["_measurement"] == "poseidon_nomad_idle_runners")
   |> filter(fn: (r) => r["_field"] == "startup_duration")
   |> filter(fn: (r) => contains(value: r["environment_id"], set: ${environment_ids:json}))
-  |> filter(fn: (r) => (not exists r.stage) or contains(value: r["stage"], set: ${stages:json}))
   |> keep(columns: ["_value", "_time", "environment_id", "stage"])
   |> aggregateWindow(every: v.windowPeriod, fn: mean, createEmpty: false)