delete the cloud jx-sandbox cluster in project jx-preprod

Delete the jx-sandbox K8s cluster within the JX GCP project (badly called jx-preprod)
https://console.cloud.google.com/kubernetes/clusters/details/us-east1-b/jx-sandbox?project=jx-preprod

A proper validation process is being set. The current PREPROD jx-sandbox) is useless as is. In all cases, the solution will likely be based on a PREPROD cluster being set up and teared down on need.

It's currently useless (and expensive) to keep the jx-sandbox cluster running while it can be set up on demand. When performing an action impacting the infrastructure, one must take a breath, thinking if it should/can/must be performed on PREPROD first. As said in the JX meeting, those use cases have to be enumerated and documented:

• which use cases do require or not a validation on a PREPROD cluster?
• in both situations, how to perform tests? (with or w/o PREPROD cluster, a test method is mandatory)