Remove ocp4-extra repo from mergeable plugin list

[yussufsh]

The ocp-power-automation/ocp4-extras repo does not require +2 hence removing the mergeable plugin from the config.

[ppc64le-cloud-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mkumatag, yussufsh

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/ppc64le-cloud/test-infra/blob/master/OWNERS)~~ [mkumatag] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ppc64le-cloud-bot]

@yussufsh: Updated the plugins configmap in namespace prow at cluster default using the following files:

• key plugins.yaml using file config/prow/plugins.yaml

In response to [this](https://github.com/ppc64le-cloud/test-infra/pull/432): >The ocp-power-automation/ocp4-extras repo does not require +2 hence removing the mergeable plugin from the config. Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[Rajalakshmi-Girish]

@yussufsh Hope the below plugins need +2 approval

ocp4-upi-powervs ocp4-upi-powervm ocp4-playbooks openshift-install-power