Clean up Viewport's forced_mouse_focus

[kitbdev]

The EditorObjectSelector::_show_popup called PopupMenu::take_mouse_focus, which called Viewport::pass_mouse_focus_to, which did nothing because the _show_popup is only called on left mouse button release and there is no mouse_focus set when the mouse is released. Since that was the only usage these methods were removed.

Viewport::pass_mouse_focus_to is the only place thatgui.forced_mouse_focus is set to true, so it is also removed.

[akien-mga]

Thanks!