[Dataset Quality] Fix flaky serverless test (elastic#192198)

## Summary

closes - elastic#192172

Fixes a flaky test introduced recently

1. Forgot to uncomment the after block
2. Missed to remove a code which was remove from stateful, but not in
serverless

x-pack/test_serverless/functional/test_suites/observability/dataset_quality/dataset_quality_details_degraded_field_flyout.ts

@@ -47,7 +47,7 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
     });
 
     after(async () => {
-      // await synthtrace.clean();
+      await synthtrace.clean();
     });
 
     describe('degraded field flyout open-close', () => {
@@ -63,10 +63,6 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
         );
 
         await PageObjects.datasetQuality.closeFlyout();
-
-        await testSubjects.missingOrFail(
-          PageObjects.datasetQuality.testSubjectSelectors.datasetQualityDetailsDegradedFieldFlyout
-        );
       });
 
       it('should open the flyout when navigating to the page with degradedField in URL State', async () => {