Remove cert manager tests

[skonto]

Which issue(s) this PR fixes:

Fixes knative/serving#15266
After this is merged I will remove all tests in knative/serving#15261.

[skonto]

/assign @dprotaso

[dprotaso]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, skonto

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[knative-prow]

@skonto: Updated the job-config configmap in namespace default at cluster default using the following files:

• key serving-main.gen.yaml using file prow/jobs/generated/knative/serving-main.gen.yaml

In response to this:

Which issue(s) this PR fixes:

Fixes knative/serving#15266
After this is merged I will remove all tests in knative/serving#15261.

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