SetEnterFocusInputArea should be removed/deprecated as it is not used

BACKGROUND

Some months ago, while implementing the accessibility support for the Launcher I realized that the Launcher didn't emit properly the signals InputArea::OnStartFocus and InputArea::OnEndFocus, as was the parent the one emitting it.

So Jay included the concept of the _enter_focus_input_area, the InputArea inside the BaseWindow that effectively would receive the keyboard focus, and Unity started to use it.

CURRENT STATE

But right now SetEnterFocusInputArea is not used at all on Nux, and although it is used on Unity (PlacesController and unityshell), the field that it sets "_enter_focus_input_area" is not used at all.

In the specific case of the Launcher, it seems that gets/free the keyboard focus using GrabKeyboard/UnGrabKeyboard, and after a quick smoke test, it seems that removing that call doesn't have any effect.

So I would suggest to remove that method, and update accordingly Unity, as it seems that is useless.