nixos/startx: send Xorg log to the default location

This partially reverts bf3d3dd19b.

I don't know why we weren't getting a default logfile back then but Xorg
definitely provides one now ($XDG_DATA_HOME for regular users and /var/log for
root, see `man Xorg`)

nixos/modules/services/x11/display-managers/startx.nix

@@ -39,6 +39,14 @@ in
       displayManager.lightdm.enable = lib.mkForce false;
     };
     systemd.services.display-manager.enable = false;
+
+    # Other displayManagers log to /dev/null because they're services and put
+    # Xorg's stdout in the journal
+    #
+    # To send log to Xorg's default log location ($XDG_DATA_HOME/xorg/), we do
+    # not specify a log file when running X
+    services.xserver.logFile = mkDefault null;
+
     # Implement xserverArgs via xinit's system-wide xserverrc
     environment.etc."X11/xinit/xserverrc".source = pkgs.writeShellScript "xserverrc" ''
       exec ${pkgs.xorg.xorgserver}/bin/X ${toString config.services.xserver.displayManager.xserverArgs} "$@"

pkgs/servers/x11/xorg/overrides.nix

@@ -765,9 +765,6 @@ self: super:
     ];
     propagatedBuildInputs = attrs.propagatedBuildInputs or [] ++ [ self.xauth ]
                          ++ lib.optionals isDarwin [ self.libX11 self.xorgproto ];
-    prePatch = ''
-      sed -i 's|^defaultserverargs="|&-logfile \"$HOME/.xorg.log\"|p' startx.cpp
-    '';
     postFixup = ''
       substituteInPlace $out/bin/startx --replace $out/etc/X11/xinit/xserverrc /etc/X11/xinit/xserverrc
     '';