Fix: Cloudwatch fix for dimension value (#16356)

Fixes autocomplete suggestion after changing dimension key

Fixes #15984

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

@@ -112,6 +112,7 @@ export class CloudWatchQueryParameterCtrl {
         query = $scope.datasource.getDimensionKeys($scope.target.namespace, $scope.target.region);
       } else if (segment.type === 'value') {
         const dimensionKey = $scope.dimSegments[$index - 2].value;
+        delete target.dimensions[dimensionKey];
         query = $scope.datasource.getDimensionValues(
           target.region,
           target.namespace,