cpuset: Delete 'builder' methods

All usage of builder pattern is convertible to cpuset.New()
with the same or fewer lines of code.

Migrate Builder.Add to a private method of CPUSet, with a comment
that it is only intended for internal use to preserve immutable
propoerty of the exported interface.

This also removes 'require' library dependency, which avoids
non-standard library usage.

test/e2e_node/podresources_test.go

@@ -522,11 +522,11 @@ func podresourcesGetAllocatableResourcesTests(ctx context.Context, cli kubeletpo
 	resp, err := cli.GetAllocatableResources(ctx, &kubeletpodresourcesv1.AllocatableResourcesRequest{})
 	framework.ExpectNoErrorWithOffset(1, err)
 	devs := resp.GetDevices()
-	b := cpuset.NewBuilder()
+	var cpus []int
 	for _, cpuid := range resp.GetCpuIds() {
-		b.Add(int(cpuid))
+		cpus = append(cpus, int(cpuid))
 	}
-	allocatableCPUs := b.Result()
+	allocatableCPUs := cpuset.New(cpus...)
 
 	if onlineCPUs.Size() == 0 {
 		ginkgo.By("expecting no CPUs reported")