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Clean up redundant DNS related codes
**What this PR does / why we need it**:
As https://github.com/kubernetes/kubernetes/pull/54773#discussion_r148904955 described, resolv.conf setup for pod is handled by `generatePodSandboxConfig()`, though we have some redundant DNS related codes in `GenerateRunContainerOptions()` which seems to have no effect.
This PR cleans up the ineffective codes and rearranges the cluster DNS unit test and hopefully it would be less confusing.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes#55201
**Special notes for your reviewer**:
cc @Random-Liu @phsiao
**Release note**:
```release-note
NONE
```
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