nixos: remove StandardOutput=syslog, StandardError=syslog lines

Since systemd 243, docs were already steering users towards using
`journal`:

https://github.com/systemd/systemd/commit/eedaf7f322a850c5d9f49346d43420423fc6f593

systemd 246 will go one step further, it shows warnings for these units
during bootup, and will [automatically convert these occurences to
`journal`](https://github.com/systemd/systemd/commit/f3dc6af20f410702beb8e45ddf77e92289fc90c7):

> [    6.955976] systemd[1]: /nix/store/hwyfgbwg804vmr92fxc1vkmqfq2k9s17-unit-display-manager.service/display-manager.service:27: Standard output type syslog is obsolete, automatically updating to journal. Please update│······················
 your unit file, and consider removing the setting altogether.

So there's no point of keeping `syslog` here, and it's probably a better
idea to just not set it, due to:

> This setting defaults to the value set with DefaultStandardOutput= in
> systemd-system.conf(5), which defaults to journal.

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

@@ -200,7 +200,6 @@ in
       KillMode = "mixed";
       IgnoreSIGPIPE = "no";
       BusName = "org.gnome.DisplayManager";
-      StandardOutput = "syslog";
       StandardError = "inherit";
       ExecReload = "${pkgs.coreutils}/bin/kill -SIGHUP $MAINPID";
       KeyringMode = "shared";

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

@@ -253,7 +253,6 @@ in
       KeyringMode = "shared";
       KillMode = "mixed";
       StandardError = "inherit";
-      StandardOutput = "syslog";
     };
 
     environment.etc."lightdm/lightdm.conf".source = lightdmConf;