chore(integration): delete outdated TODO comment

The TODO comment was in regards to allowing graphdriver plugins to
provide their own ContainerFS implementations. The ContainerFS interface
has been removed from Moby, so there is no longer anything which needs
to be figured out.

Signed-off-by: Cory Snider <csnider@mirantis.com>

integration/plugin/graphdriver/external_test.go

@@ -213,7 +213,6 @@ func setupPlugin(t *testing.T, ec map[string]*graphEventsCounter, ext string, mu
 			return
 		}
 
-		// TODO @gupta-ak: Figure out what to do here.
 		dir, err := driver.Get(req.ID, req.MountLabel)
 		if err != nil {
 			respond(w, err)