[knative][flake] Removal of the whole app leaves the imagestream

Removing the application though the UI, that contains Knative service, that was deployed thought the UI form, may leave out the image stream.

That omitted image stream will make redeploying of the same application impossible. Also, this omitted image stream makes the UI still display the application, despite it being removed.

4.12.0

flaky

1. Deploy knative service via UI form, with specifying application grouping
2. Delete the whole application

The image stream for the kservice is left out

All elements labeled `app.kubernetes.io/part-of=app-name` in selected namespace should be removed