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/LokiQueryField.tsx

@@ -3,21 +3,15 @@ import { LokiQueryFieldForm, LokiQueryFieldFormProps } from './LokiQueryFieldFor
 import { useLokiSyntax } from './useLokiSyntax';
 import LokiLanguageProvider from '../language_provider';
 
-export const LokiQueryField: FunctionComponent<LokiQueryFieldFormProps> = ({
-  datasource,
-  datasourceStatus,
-  ...otherProps
-}) => {
+export const LokiQueryField: FunctionComponent<LokiQueryFieldFormProps> = ({ datasource, ...otherProps }) => {
   const { isSyntaxReady, setActiveOption, refreshLabels, ...syntaxProps } = useLokiSyntax(
     datasource.languageProvider as LokiLanguageProvider,
-    datasourceStatus,
     otherProps.absoluteRange
   );
 
   return (
     <LokiQueryFieldForm
       datasource={datasource}
-      datasourceStatus={datasourceStatus}
       syntaxLoaded={isSyntaxReady}
       /**
        * setActiveOption name is intentional. Because of the way rc-cascader requests additional data