Automatic merge from submit-queue
Remove the kube-discovery binary from the tree
**What this PR does / why we need it**:
kube-discovery was a temporary solution to implementing proposal: https://github.com/kubernetes/community/blob/master/contributors/design-proposals/bootstrap-discovery.md
However, this functionality is now gonna be implemented in the core for v1.6 and will fully replace kube-discovery:
- https://github.com/kubernetes/kubernetes/pull/36101
- https://github.com/kubernetes/kubernetes/pull/41281
- https://github.com/kubernetes/kubernetes/pull/41417
So due to that `kube-discovery` isn't used in any v1.6 code, it should be removed.
The image `gcr.io/google_containers/kube-discovery-${ARCH}:1.0` should and will continue to exist so kubeadm <= v1.5 continues to work.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
**Special notes for your reviewer**:
**Release note**:
```release-note
Remove cmd/kube-discovery from the tree since it's not necessary anymore
```
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