Remove remaining code for deprecated openapi paths

[roycaihw]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
Followup of #73148. Remove remaining fallback logic in discovery client and aggregator downloader.

Does this PR introduce a user-facing change?:

kube-apiserver now only aggregates openapi schemas from `/openapi/v2` endpoints of aggregated API servers. The fallback to aggregate from `/swagger.json` has been removed. Ensure aggregated API servers provide schema information via `/openapi/v2` (available since v1.10).

/cc @liggitt @mbohlool

[roycaihw]

/retest

[k8s-ci-robot]

@roycaihw: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-kubernetes-e2e-kops-aws	dd3ce64	link	/test pull-kubernetes-e2e-kops-aws

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[liggitt]

/retest
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• staging/src/k8s.io/client-go/OWNERS [liggitt]
• staging/src/k8s.io/kube-aggregator/OWNERS [liggitt]

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