screenshots of app template were still deleted when delete screenshots but not to save

Created by: junotx

Describe the bug

Screenshots of app template were still deleted when edit template and click "delete all" to delete screenshots but not to save

Versions used(KubeSphere/Kubernetes)

KubeSphere: latest(20210421)

Expected behavior

Screeshots should be deleted if and only if the edit is saved

/kind bug /assign @zheng1