kubelet: remove background updating thread in RuntimeCache

This feature is no longer useful pods don't sync as often. For batch
creation/deletions/syncs, the cache will be up-to-date for most pods since it
will be updated frequently. For other cases, continue updating for two more
seconds don't usually help, as temporal locality doesn't hold across pod syncs.

pkg/kubelet/container/runtime_cache_test.go

@@ -86,27 +86,3 @@ func TestForceUpdateIfOlder(t *testing.T) {
 		t.Errorf("expected %#v, got %#v", newpods, actual)
 	}
 }
-
-func TestUpdatePodsOnlyIfNewer(t *testing.T) {
-	runtime := &FakeRuntime{}
-	cache := newTestRuntimeCache(runtime)
-
-	// Cache new pods with a future timestamp.
-	newpods := []*Pod{{ID: "1111"}, {ID: "2222"}, {ID: "3333"}}
-	cache.Lock()
-	cache.pods = newpods
-	cache.cacheTime = time.Now().Add(20 * time.Minute)
-	cache.Unlock()
-
-	// Instruct runime to return a list of old pods.
-	oldpods := []*Pod{{ID: "1111"}}
-	runtime.PodList = oldpods
-
-	// Try to update the cache; the attempt should not succeed because the
-	// cache timestamp is newer than the current time.
-	cache.updateCacheWithLock()
-	actual := cache.getCachedPods()
-	if !reflect.DeepEqual(newpods, actual) {
-		t.Errorf("expected %#v, got %#v", newpods, actual)
-	}
-}