kcms/wallpaper: Avoid storing screen as raw pointer
Original commit message(s)
- kcms/wallpaper: Avoid duplicating screen tracking
m_screens is just QGuiApplication::screens. There's no point us tracking it separately
- kcms/wallpaper: Avoid storing screen as raw pointer
The screen is not owned by the WallpaperModule, nor is it cleaned up in screenRemoved.
m_selectedScreen is already guarded in most places.