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/loki/components/LokiQueryEditor.tsx

@@ -3,7 +3,7 @@ import React, { memo } from 'react';
 
 // Types
 import { AbsoluteTimeRange } from '@grafana/data';
-import { QueryEditorProps, DataSourceStatus } from '@grafana/ui';
+import { QueryEditorProps } from '@grafana/ui';
 import { LokiDatasource } from '../datasource';
 import { LokiQuery } from '../types';
 import { LokiQueryField } from './LokiQueryField';
@@ -30,8 +30,6 @@ export const LokiQueryEditor = memo(function LokiQueryEditor(props: Props) {
 
   const { isSyntaxReady, setActiveOption, refreshLabels, ...syntaxProps } = useLokiSyntax(
     datasource.languageProvider,
-    // TODO maybe use real status
-    DataSourceStatus.Connected,
     absolute
   );
 
@@ -39,7 +37,6 @@ export const LokiQueryEditor = memo(function LokiQueryEditor(props: Props) {
     <div>
       <LokiQueryField
         datasource={datasource}
-        datasourceStatus={DataSourceStatus.Connected}
         query={query}
         onChange={onChange}
         onRunQuery={onRunQuery}