Remove exitViro API from Scene Navigator
Remove exitViro API from Scene Navigator

- Due to the lack of stability of tearing down GVR layouts in iOS,
I've removed exitViro from the scene navigator (note onExitViro
callbacks are still there for when the user clicks the X button),
until a point where GVR provides the proper teardown calls.


Former-commit-id: 8be9fe35f29a7f526552f26051f1c17ed8f3dcb1
viromedia/viro,viromedia/viro,viromedia/viro,viromedia/viro