Automatic merge from submit-queue (batch tested with PRs 53743, 53564). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
kubelet: remove the --network-plugin-dir flag
**What this PR does / why we need it**:
This flag has been replaced with `--cni-bin-dir`, and has been deprecated in Kubernetes 1.7.
It is safe to remove in Kubernetes 1.9 according to the deprecation policy.
**Which issue this PR fixes**: fixes#46410
**Special notes for your reviewer**:
/assign @mtaufen @freehan @dchen1107
**Release note**:
```release-note
Remove the --network-plugin-dir flag.
```