ScreenView visibleProperty should not be featured

[pixelzoom]

Noticed when migrating overrides.js into code for #331.

In natural-selection-overrides.js, I see that the visibleProperty of both ScreenViews is featured:

    "naturalSelection.introScreen.view.visibleProperty": {
      "phetioFeatured": true
    },
    "naturalSelection.labScreen.view.visibleProperty": {
      "phetioFeatured": true
    }

This seems odd, because

• these visiblePropetyelements are phetioReadOnly: true
• no other sims feature this Property (search all overrides.js for "Screen.view.visibleProperty" to confirm)

@amanda-phet was this intentional, or is this a bug? I recommend not featuring these elements.

[amanda-phet]

I have no recollection of featuring this, and can't understand why it's featured (being read-only). Please un-feature!

[pixelzoom]

Done in the above commits. view.visibleProperty is now phetioFeatured: false for both screens.