test/e2e: Delete test namespaces asynchronously

Now that namespace deletion is reliable, use the suite tear down
to catch non terminated namespaces and stop waiting within each test
for deletion.

Serial tests that need a clean set of namespaces must use the
appropriate flag to control whether they wait for clean namespaces
on startup.

test/e2e/framework/framework.go

@@ -303,11 +303,7 @@ func (f *Framework) AfterEach() {
 		if TestContext.DeleteNamespace && (TestContext.DeleteNamespaceOnFailure || !ginkgo.CurrentGinkgoTestDescription().Failed) {
 			for _, ns := range f.namespacesToDelete {
 				ginkgo.By(fmt.Sprintf("Destroying namespace %q for this suite.", ns.Name))
-				timeout := DefaultNamespaceDeletionTimeout
-				if f.NamespaceDeletionTimeout != 0 {
-					timeout = f.NamespaceDeletionTimeout
-				}
-				if err := deleteNS(f.ClientSet, f.DynamicClient, ns.Name, timeout); err != nil {
+				if err := f.ClientSet.CoreV1().Namespaces().Delete(ns.Name, nil); err != nil {
 					if !apierrors.IsNotFound(err) {
 						nsDeletionErrors[ns.Name] = err
 					} else {