nixos/desktop-manager: Don't fall back to black background

Previously if ~/.background-image wasn't present, the background would
be set to black, which would override what the user could
set in e.g. services.xserver.windowManager.i3.extraSessionCommands

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

@@ -74,9 +74,6 @@ in
           + optionalString (needBGCond d) ''
             if [ -e $HOME/.background-image ]; then
               ${pkgs.feh}/bin/feh --bg-${cfg.wallpaper.mode} ${optionalString cfg.wallpaper.combineScreens "--no-xinerama"} $HOME/.background-image
-            else
-              # Use a solid black background as fallback
-              ${pkgs.xorg.xsetroot}/bin/xsetroot -solid black
             fi
           '';
         });