Automatic merge from submit-queue
kubeadm: Remove the validate phase as it's not needed
**What this PR does / why we need it**:
This validation code was added in v1.4 as a way to remove flakiness between deploying the control plane in Static Pods and deploying kube-discovery as a Deployment.
That isn't the case anymore and we're not experiencing such flakiness, as we're using other methods like checking `/healthz` to determine a healthy control plane before proceeding.
https://github.com/kubernetes/kubernetes/pull/43881 removed this logic from `kubeadm init` to having it as a phase. But that phase isn't needed or used in any way, so now I'm removing it here.
**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**:
Targets v1.8
**Release note**:
```release-note
NONE
```
@kubernetes/sig-cluster-lifecycle-pr-reviews @timothysc @mikedanese
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