Restore help for --service-account-private-key-file flag, avoid incorrect deprecation notice

[liggitt]

https://github.com/kubernetes/kubernetes/pull/60270/files#diff-fc9db90cc6b68d7c3ca838a8512447e9R61 incorrectly merged the ServiceAccountKeyFile flag used by the cloud controller manager (deprecated and scheduled to be removed in 1.10), and the one used by the kube-controller-manager (not deprecated, still supported)

This made the help for the supported flag disappear from 1.11 help output.

This PR:

• moves the option to the service account controller options
• removes the incorrect deprecation notice
• drops the option from the cloud-controller-manager

The deprecated `--service-account-private-key-file` flag has been removed from the cloud-controller-manager. The flag is still present and supported in the kube-controller-manager.

[liggitt]

/assign @sttts

moving into the milestone to avoid marking a critical flag deprecated and hiding help for it

/milestone v1.11
/kind bug
/sig api-machinery
/priority critical-urgent
/status approved-for-milestone

[dims]

/assign @deads2k

[k8s-github-robot]

[MILESTONENOTIFIER] Milestone Pull Request: Up-to-date for process

@deads2k @liggitt @sttts

Pull Request Labels

• sig/api-machinery: Pull Request will be escalated to these SIGs if needed.
• priority/critical-urgent: Never automatically move pull request out of a release milestone; continually escalate to contributor and SIG through all available channels.
• kind/bug: Fixes a bug discovered during the current release.

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[deads2k]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: deads2k, liggitt

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Needs approval from an approver in each of these files:

• cmd/controller-manager/OWNERS [deads2k]
• cmd/kube-controller-manager/OWNERS [deads2k]
• pkg/apis/componentconfig/OWNERS [liggitt]

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[k8s-github-robot]

/test all [submit-queue is verifying that this PR is safe to merge]

[k8s-github-robot]

Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions here.