whonix-gw-kde-desktop-conf: no longer use custom wallpaper (mountain mist) for Whonix-Gateway. Only use wallpapers from Debian repository for security reasons. (https://github.com/Whonix/Whonix/issues/318) Will now default to KDE's default wallpaper. (Thanks to https://github.com/HulaHoopWhonix for suggesting it)