Small code cleanup within scenes

[Julius2342]

Description:

• should_poll of base class already returns False
• there is no is_on within Scene

[MartinHjelmare]

Thanks!

[balloob]

History note: we used to be able to turn off scenes. I wrote some complicated logic that would allow a scene to be "undone". It would try to change the states back to the state before the scene was turned on. A scene was considered "on" when the devices would match state/attributes with the scene.

However, this would only work if the device would report the state exactly as we wanted it to set from the initial scene. I quickly realized that this would not work with things like X, Y color for Hue. The reported value always differs from the command.

I decided to remove the feature but it looks like some things stuck around. Thanks for cleaning this up.