vkcube: fix crash on exit from double delete

Author: richard-lunarg

cube/macOS/cube/DemoViewController.m

@@ -89,7 +89,6 @@ static CVReturn DisplayLinkCallback(CVDisplayLinkRef displayLink, const CVTimeSt
  demo_run(demo);
  if (demo->quit) {
  CVDisplayLinkStop(displayLink);
- CVDisplayLinkRelease(displayLink);
  }
  return kCVReturnSuccess;
 }

cube/macOS/cubepp/DemoViewController.mm

@@ -88,7 +88,6 @@ static CVReturn DisplayLinkCallback(CVDisplayLinkRef displayLink, const CVTimeSt
  demo->run();
  if (demo->quit) {
  CVDisplayLinkStop(displayLink);
- CVDisplayLinkRelease(displayLink);
  }
  return kCVReturnSuccess;
 }