don't merge hard coded region list

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

@@ -75,7 +75,7 @@ export class CloudWatchConfigCtrl {
         return ds.getRegions();
       })
       .then(regions => {
-        this.regions = _.uniq(this.regions.concat(_.map(regions, 'value')));
+        this.regions = _.map(regions, 'value');
       }, err => {
         console.error('failed to call ec2:DescribeRegions, please check iam setting');
       });