Delete TODO in image_gc_manager

I think the TODO here may have actually been unnecessary. There isn't a
ton of interest around merging
kubernetes#87425, which contains a
fix. Delete the TODO so we don't devote time to working on this area in
the future.

Author: mattjmcnaughton

pkg/kubelet/images/image_gc_manager.go

@@ -185,7 +185,6 @@ func (im *realImageGCManager) Start() {
 	}, 5*time.Minute, wait.NeverStop)
 
 	// Start a goroutine periodically updates image cache.
-	// TODO(random-liu): Merge this with the previous loop.
 	go wait.Until(func() {
 		images, err := im.runtime.ListImages()
 		if err != nil {