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