task graph test: do not wait after cancelation

Given correct synchronization I do not see any value in the `Sleep`, it just makes the test much slower:

```console
$ go test --count 10 --run TestRunGraph/cancelation_without_task_errors  ./pkg/payload/
ok  	github.com/openshift/cluster-version-operator/pkg/payload	10.041s

$ git checkout ocpbugs-22442-test-run-graph-mid-task-cancellation-flake
...
$ go test --count 10 --run TestRunGraph/cancellation_without_task_errors  ./pkg/payload/
ok  	github.com/openshift/cluster-version-operator/pkg/payload	0.043s
```

Author: petr-muller

pkg/payload/task_graph_test.go

@@ -949,7 +949,6 @@ func TestRunGraph(t *testing.T) {
 			callbacks: map[string]callbackFn{
 				"a": func(t *testing.T, name string, ctx context.Context, cancelFn func()) error {
 					cancelFn()
-					time.Sleep(time.Second)
 					return nil
 				},
 				"*": func(t *testing.T, name string, ctx context.Context, cancelFn func()) error {