fix(tests): snackbar ref has unnecessary observable.of (#2298)

src/lib/snack-bar/snack-bar-ref.ts

@@ -5,7 +5,6 @@ import {MdSnackBarContainer} from './snack-bar-container';
 
 // TODO(josephperrott): Implement onAction observable.
 
-Observable.of([]);
 /**
  * Reference to a snack bar dispatched from the snack bar service.
  */