Fix Grafana dashboard datasource for file-provisioned mode

File-provisioned dashboards do not resolve ${DS_PROMETHEUS} template
variables. Remove uid from all panel datasource references so Grafana
auto-selects the default Prometheus datasource.

Author: ev-shindin

deploy/grafana/benchmark-dashboard.json

@@ -23,7 +23,7 @@
       "title": "Deployment Replicas",
       "type": "timeseries",
       "gridPos": { "h": 8, "w": 12, "x": 0, "y": 0 },
-      "datasource": { "type": "prometheus", "uid": "${DS_PROMETHEUS}" },
+      "datasource": { "type": "prometheus" },
       "fieldConfig": {
         "defaults": {
           "color": { "mode": "palette-classic" },
@@ -59,7 +59,7 @@
       "title": "WVA Desired Ratio",
       "type": "timeseries",
       "gridPos": { "h": 8, "w": 12, "x": 12, "y": 0 },
-      "datasource": { "type": "prometheus", "uid": "${DS_PROMETHEUS}" },
+      "datasource": { "type": "prometheus" },
       "fieldConfig": {
         "defaults": {
           "color": { "mode": "palette-classic" },
@@ -88,7 +88,7 @@
       "title": "KV Cache Usage",
       "type": "timeseries",
       "gridPos": { "h": 8, "w": 12, "x": 0, "y": 8 },
-      "datasource": { "type": "prometheus", "uid": "${DS_PROMETHEUS}" },
+      "datasource": { "type": "prometheus" },
       "fieldConfig": {
         "defaults": {
           "color": { "mode": "palette-classic" },
@@ -131,7 +131,7 @@
       "title": "Queue Depth (Requests Waiting)",
       "type": "timeseries",
       "gridPos": { "h": 8, "w": 12, "x": 12, "y": 8 },
-      "datasource": { "type": "prometheus", "uid": "${DS_PROMETHEUS}" },
+      "datasource": { "type": "prometheus" },
       "fieldConfig": {
         "defaults": {
           "color": { "mode": "palette-classic" },
@@ -165,7 +165,7 @@
       "title": "Scaling Activity",
       "type": "timeseries",
       "gridPos": { "h": 8, "w": 24, "x": 0, "y": 16 },
-      "datasource": { "type": "prometheus", "uid": "${DS_PROMETHEUS}" },
+      "datasource": { "type": "prometheus" },
       "fieldConfig": {
         "defaults": {
           "color": { "mode": "palette-classic" },
@@ -202,21 +202,7 @@
   ],
   "schemaVersion": 39,
   "tags": ["benchmark", "wva", "autoscaling"],
-  "templating": {
-    "list": [
-      {
-        "current": { "selected": false, "text": "Prometheus", "value": "Prometheus" },
-        "hide": 0,
-        "includeAll": false,
-        "name": "DS_PROMETHEUS",
-        "options": [],
-        "query": "prometheus",
-        "refresh": 1,
-        "regex": "",
-        "type": "datasource"
-      }
-    ]
-  },
+  "templating": { "list": [] },
   "time": { "from": "now-30m", "to": "now" },
   "timepicker": {},
   "timezone": "utc",