Changes for easier porting to VictoriaMetrics datasource

[wscheicher]

Since there is now a VictoriaMetrics Datasource, i suggest some minor tweaks to allow easier porting:

1. remove "type": "prometheus", where not needed.
   I noticed that wherever there is the datasource referenced via variable, it isn't necessary to specifiy it.
   ( sed command i used to strip that: sed -i '/"type": "prometheus",/d' )

2. in the templating part, there the datasource selector seems to use the wrong variable name? It seems unreferenced.
   I had to change "name": "datasource", to "name": "DS_PROMETHEUS", for it to work. But i don't know if this is the way to go.

3. The "__inputs" block seems to be not needed if the datasource selector in the templating part works.

If this is done, then a port to VictoriaMetrics only requires two small changes:
The plugin in the "__requires":

  "__requires": [

...

    {
      "type": "datasource",
      "id": "victoriametrics-metrics-datasource",
      "name": "VictoriaMetrics",
      "version": ""
    },

And the datasource query in "templating":

  "templating": {
    "list": [
      {
        "current": {},
        "includeAll": false,
        "label": "Datasource",
        "name": "DS_PROMETHEUS",
        "options": [],
        "query": "victoriametrics-metrics-datasource",
        "refresh": 1,
        "regex": "",
        "type": "datasource"
      },

[wscheicher]

Update: I just figured out: the version from grafana didn't include the latest commits, where, there are already "datasource" declarations being removed.
Just not all of them.
I tested and figured out: all the remaining declarations seem to be removable.

And the variable name also has been fixed.
(But wouldn't matter anymore if all "datasource" declarations are removed)