Remove the default wallpaper (for Dell server RDP access)

We recommend Dell servers to our clients. However for some bizarre reason they set the default wallpaper to their own high colour Dell wallpaper that is highly graded.

Unfortunately this has the side effect of making it very slow to access the server via RDP when connecting, as it has to transmit this wallpaper even when the "Wallpaper" option is switched off in the RDP client.