Also, remove `plnkr.json` for `service-worker-getting-started` guide,
since it is not used and ServiceWorker cannot work correctly in
plnkr/stackblitz anyway (e.g. no build step to re-compute hashes).
A zipper might be useful and can be added in a subsequent PR, but it is
currently broken (e.g. no dependency on `@angular/service-worker`).
PR Close#20165
