fix superfluous `retry.RetryOnConflict` in `decomission_controller`

[notandy]

example code to be tested for decomission_controller.go:86

		return ctrl.Result{}, k8sclient.IgnoreNotFound(retry.RetryOnConflict(retry.DefaultRetry, func() error {
			node := &corev1.Node{}
			if err := r.Get(ctx, req.NamespacedName, node); err != nil {
				return err
			}
			nodeBase := node.DeepCopy()
			if controllerutil.AddFinalizer(node, decommissionFinalizerName) {
				return r.Patch(ctx, node, k8sclient.MergeFromWithOptions(
					nodeBase, k8sclient.MergeFromWithOptimisticLock{}))
			}
			return nil
		}))