Revert "nixos/pantheon: install nixos wallpaper"

This reverts commit 5100e4f.

Fixes NixOS#100293
Though it's only a workaround for now.
See elementary/settings-desktop#246 (comment)
We trigger the broken scenario where we have two subdirectories. Reverting
that commit undoes this.

nixos/modules/services/x11/desktop-managers/pantheon.nix

@@ -180,7 +180,6 @@ in
         gtk3.out
         hicolor-icon-theme
         lightlocker
-        nixos-artwork.wallpapers.simple-dark-gray
         onboard
         qgnomeplatform
         shared-mime-info