Drop CI config for knative-extensions/net-certmanager

[ReToCode]

Changes

• Remove archived knative-extensions/net-certmanager repository

Partially knative/community#1563

[dsimansk]

/approve
/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dsimansk, ReToCode

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[knative-prow]

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

• key net-certmanager-main.gen.yaml using file ``
• key net-certmanager-release-1.13.gen.yaml using file ``
• key net-certmanager-release-1.14.gen.yaml using file ``

In response to this:

Changes

• Remove archived knative-extensions/net-certmanager repository

Partially knative/community#1563

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