Remove upgrade controller

[anlandu]

Describe the solution you'd like
Now that the bug that rendered the upgrade controller necessary has been fixed, I think the upgrade controller can be safely removed (unless support for k8s <=1.13 is still required). Any thoughts on removing it? Have seen a few instances where customers are concerned thinking errors from this loop are causing impact. This is compounded by the fact that GET it does fetches not just v1alpha1 but all resources (due to the hub and spoke architecture), meaning that on every controller pod startup every GK resource is PUT again even if they're all stored properly as v1.

Environment:

• Gatekeeper version: 3.17.1
• Kubernetes version: (use kubectl version):