"verification-mode" field should be removed from pipelines spec

[jkandasa]

Originally verification-mode from pipelines spec was removed in https://github.com/tektoncd/operator/pull/1481, but upgrade blocked with the following error,

{"level":"info","logger":"tekton-operator-webhook","caller":"webhook/admission.go:151","msg":"remote admission controller audit annotations=map[string]string(nil)","commit":"8e8c163","knative.dev/kind":"operator.tekton.dev/v1alpha1, Kind=TektonConfig","knative.dev/namespace":"","knative.dev/name":"config","knative.dev/operation":"UPDATE","knative.dev/resource":"operator.tekton.dev/v1alpha1, Resource=tektonconfigs","knative.dev/subresource":"","knative.dev/userinfo":"system:serviceaccount:openshift-operators:openshift-pipelines-operator","admissionreview/uid":"958be8bd-9d63-4b95-aa7d-9285684ff86c","admissionreview/allowed":false,"admissionreview/result":"&Status{ListMeta:ListMeta{SelfLink:,ResourceVersion:,Continue:,RemainingItemCount:nil,},Status:Failure,Message:mutation failed: cannot decode incoming old object: json: unknown field \"verification-mode\",Reason:BadRequest,Details:nil,Code:400,}"}

hence verification-mode field added back to pipelines spec with the PR => https://github.com/tektoncd/operator/pull/1498

verification-mode should be removed, keep this ticket open until verification-mode removed.