mpd: use systemd journal instead of syslog

MPD is using syslog for its logging output, while it could directly
log to systemd's journal, as this daemon is primarily used as a
systemd user service. This change makes MPD log to standard output,
which is captured by systemd.

See https://github.com/NixOS/nixpkgs/pull/57608, which does the same
thing to NixOS's MPD service.

modules/services/mpd.nix

@@ -16,7 +16,6 @@ let
     ''}
     state_file          "${cfg.dataDir}/state"
     sticker_file        "${cfg.dataDir}/sticker.sql"
-    log_file            "syslog"
 
     ${optionalString (cfg.network.listenAddress != "any")
       ''bind_to_address "${cfg.network.listenAddress}"''}