Remove the temporary fix for pre-1.0 mirror pods

[yujuhong]

The fix was introduced to fix #15960 for pre-1.0 pods. It should be safe to remove
this fix now.

[k8s-reviewable]

This change is 

[lavalamp]

/lgtm
/approve

But I think this needs an action required release note on the off chance that someone still has 1.0 kubelets running.

[yujuhong]

But I think this needs an action required release note on the off chance that someone still has 1.0 kubelets running.

"Pre-1.0" kubelet.... ;)

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

The following people have approved this PR: lavalamp, yujuhong

Needs approval from an approver in each of these OWNERS Files:

• pkg/api/OWNERS [lavalamp]
• pkg/api/v1/OWNERS [lavalamp]

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[lavalamp]

hm, the old code says things like, "// TODO: Remove this code after we drop support for v1.0 kubelets."

[k8s-github-robot]

@k8s-bot test this [submit-queue is verifying that this PR is safe to merge]

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 40289, 40877, 40879, 39972, 40942)