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Standard output type syslog is obsolete, automatically updating to journal.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/tests/virtualbox.nix

@@ -198,7 +198,6 @@ let
       systemd.services."vboxtestlog-${name}@" = {
         description = "VirtualBox Test Machine Log For ${name}";
         serviceConfig.StandardInput = "socket";
-        serviceConfig.StandardOutput = "syslog";
         serviceConfig.SyslogIdentifier = "GUEST-${name}";
         serviceConfig.ExecStart = "${pkgs.coreutils}/bin/cat";
       };