Use default pmOS wallpaper again for Phosh and (non-mobile) GNOME

Setting a different wallpaper than the default one makes sense for GNOME Shell on Mobile, because launchers (icons, text) are drawn directly on the wallpaper and it is too hard to read them with the default pmOS wallpaper.

However, this is not the case with Phosh and regular GNOME, so let's use the pmOS wallpaper there again for more consistency with the other UIs. It just looks awesome, when e.g. plugging an external monitor and seeing it on a big screen :D

I've verified in QEMU that it sets the expected wallpaper in Phosh, GNOME, GNOME Mobile.