CloudWatch: Fix alerting for queries with Id (using GetMetricData) (#17899)

This commit addresses half of #13749 by making sure GetMetricData 
works for alerting. Math Expressions (compound metrics) will still not 
work for alerting, this would require a bigger refactoring of Grafana's 
alerting service. However, with this commit at least alerting for basic 
metrics with non empty query Id will work.

Fixes half of #13749

public/app/plugins/datasource/cloudwatch/query_parameter_ctrl.ts

@@ -17,7 +17,6 @@ export class CloudWatchQueryParameterCtrl {
       target.region = target.region || 'default';
       target.id = target.id || '';
       target.expression = target.expression || '';
-      target.returnData = target.returnData || false;
       target.highResolution = target.highResolution || false;
 
       $scope.regionSegment = uiSegmentSrv.getSegmentForValue($scope.target.region, 'select region');