Automatic merge from submit-queue (batch tested with PRs 48295, 48298, 47339, 44910, 48037)
kubeadm: Remove v1.6 version gates, cleanup unused code, etc.
**What this PR does / why we need it**:
- Removes v1.6 version gates and requires a control plane version of v1.7.0 and above
- Removes unused/unnecessary functions that got freed up as a consequence of that
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
Fixes: kubernetes/kubeadm#327
**Special notes for your reviewer**:
This PR targets v1.8, can be merged first when the code freeze is lifted
**Release note**:
```release-note
NONE
```
@kubernetes/sig-cluster-lifecycle-pr-reviews @timothysc @mikedanese @pipejakob
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