Omit NotFound when cleaning up the Affinity Assistant

The PipelineRun reconciler cleanup the affinity assistant
when the PipelineRun is completed. If the cleanup-function
is called more than once, the DELETE request will return
a NotFound response. It does not make sense to return NotFound
responses as an error, since this is what we want to achieve.

(cherry picked from commit 204a403)

pkg/reconciler/pipelinerun/affinity_assistant.go

@@ -89,7 +89,7 @@ func (c *Reconciler) cleanupAffinityAssistants(pr *v1beta1.PipelineRun) error {
 	for _, w := range pr.Spec.Workspaces {
 		if w.PersistentVolumeClaim != nil || w.VolumeClaimTemplate != nil {
 			affinityAssistantStsName := getAffinityAssistantName(w.Name, pr.Name)
-			if err := c.KubeClientSet.AppsV1().StatefulSets(pr.Namespace).Delete(affinityAssistantStsName, &metav1.DeleteOptions{}); err != nil {
+			if err := c.KubeClientSet.AppsV1().StatefulSets(pr.Namespace).Delete(affinityAssistantStsName, &metav1.DeleteOptions{}); err != nil && !apierrors.IsNotFound(err) {
 				errs = append(errs, fmt.Errorf("failed to delete StatefulSet %s: %s", affinityAssistantStsName, err))
 			}
 		}