Explore: Remove datasource testing on selector (#19910)

* Explore: Remove datasource testing on selector

- datasource testing gets in the way of fast query iteration: switching
between datasources can take seconds
- it should not be explore's duty to test datasources in the first place
- removed the concept of datasourceError in Explore, should not be its
concern
- datasource erorrs will express themselves in query errors just fine
- connection errors are still bubbled up
- removed reconnection logic from explore, should not be its concern
- missing labels in loki are still "visible" via an empty label selector
- Loki and Prometheus treated connection errors differently than other
datasources, making sure to pass through the original error message

* Show datasource error in query field for prom/loki/influx

* Removed connection test case, fixed disabled state

public/app/plugins/datasource/prometheus/components/__snapshots__/PromQueryEditor.test.tsx.snap

@@ -9,7 +9,6 @@ exports[`Render PromQueryEditor with basic options should render 1`] = `
         "getPrometheusTime": [MockFunction],
       }
     }
-    datasourceStatus={0}
     history={Array []}
     onChange={[Function]}
     onRunQuery={[Function]}