Delete Poolkeper

[meysholdt]

Is your feature request related to a problem? Please describe

The component poolkeeper is no longer needed. To reduce complexity and prevent future toil, we should delete it.

1. The functionality from poolkeeper is needed less because we discontinue preview envs in core-dev.
2. The functionality of poolkeeper can be achieved in simpler ways:

• Ensure there is at least one node in the nodepool? Set min node count.
• Ensure kube-system pods don't block cluster-scale-down? Define pod disruption budget.

Describe the behaviour you'd like

Delete poolkeeper from this git repo and the core-dev cluster.

[geropl]

Awesome that we found better solutions for these problems - let's remove that cruft! 💪