ksonnet: remove package

The team behind ksonnet was aquired by VMware, and they decided to end
work on ksonnet:

From https://blogs.vmware.com/cloudnative/2019/02/05/welcoming-heptio-open-source-projects-to-vmware:

> Prior to the acquisition, Heptio had been shifting focus and resources
> away from ksonnet; with the acquisition, we felt it was the right time
> to rethink our investment in ksonnet. As a result, work on ksonnet will
> end and the GitHub repositories will be archived. It’s extremely
> difficult to step back from a project we have worked so hard on, but
> we’re excited about our new ideas and vision for changing how developers
> experience the Kubernetes and cloud native ecosystems.