kubeadm: remove learner-mode test job

[neolit123]

The feature gate became GA.

• [1.32]kubeadm: promote etcd learner mode to GA  kubernetes#126374
• RFE: use learner mode for joining etcd members kubeadm#1793
• kinder: remove learner-mode workflow kubeadm#3096

[neolit123]

/cc @pacoxu

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: neolit123

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[pacoxu]

/lgtm

[k8s-ci-robot]

@neolit123: Updated the job-config configmap in namespace default at cluster test-infra-trusted using the following files:

• key kubeadm-kinder-learner-mode.yaml using file ``

In response to this:

The feature gate became GA.

• [1.32]kubeadm: promote etcd learner mode to GA  kubernetes#126374
• RFE: use learner mode for joining etcd members kubeadm#1793
• kinder: remove learner-mode workflow kubeadm#3096

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