Remove Kubernetes onboarding tutorial samples

[willdavsmith]

Summary

Remove the unused demo Helm chart and Kubernetes Guestbook fixtures for the obsolete annotation-based onboarding tutorials. Preserve the demo application's source, Bicep samples, and all supported sample workflows.

Reason for change

The tutorials that consumed these fixtures were removed from docs edge in radius-project/docs#1897. The fixtures are baseline Kubernetes manifests; the deleted tutorials added the obsolete Deployment annotations and Recipe CRD examples incrementally. Historical docs references were exclusive to those tutorials, and neither current docs nor samples readmes, indexes, scripts, or workflows reference these fixtures.

Related to radius-project/radius#12952 and radius-project/radius#12204.

This cleanup targets edge only; it does not remove general Helm/Kubernetes support, Radius recipes, or supported application/environment labels.

How to test

• git diff --check passes.
• Repository-wide searches find no remaining references to the deleted fixture paths, Guestbook images, redis-secret, or removed tutorial URLs.
• Confirmed all changes are deletions in the two fixture directories (10 files, 183 lines); demo Bicep/source, build/test workflows, and application-label selectors are unchanged.
• No application runtime changes; no cloud deployments or unrelated test suites run.

File change summary

File	Summary of change
samples/demo/Chart/{Chart.yaml,values.yaml,templates/app.yaml}	Remove the three unused Helm onboarding tutorial files.
samples/kubernetes/guestbook/README.md and deploy/*.yaml	Remove the Guestbook tutorial README and six unused Deployment/Service manifests.