[cdk-pipelines] stack cleanup

Following this example [here](https://aws.amazon.com/blogs/developer/cdk-pipelines-continuous-delivery-for-aws-cdk-applications/)

```
npx cdk deploy \
--profile account1-profile \
CdkpipelinesDemoPipelineStack
```

The above command creates a pipeline stack and then the pipeline creates an application stack.

```
npx cdk destroy \
--profile account1-profile \
CdkpipelinesDemoPipelineStack
```

Only delete the pipeline stack but not the application stack created earlier.

Reproduce the example's `npx cdk deploy` and `npx cdk destroy` commands with the `CdkpipelinesDemoPipelineStack` and observe which application resources remain. The issue names no source files or tests, so trace the pipeline's application-stack cleanup behavior; done means the expected stack lifecycle is implemented or the behavior is clearly resolved.

Written by the indexing model from the issue text.