Update deletion exercise for new k8s-based infra

[zanderhavgaard]

The deletion exercise behaves differently on the new k8s-based infra. When you run rm -rf / in a container, it will sit for a long time trying to delete virtual things in /sys.

We should update the exercise to suggest deleting directories in the / directory like "try to remove /bin, what happened?", "try to remove /usr/ or /etc, what happened now?"

The slides should be updated to reflect these changes as well.