Don't run user services as user.

manifests/user_instance.pp

@@ -57,7 +57,6 @@ define mpd::user_instance (
   exec { "Enable user ${user} mpd instance":
     command => "systemctl enable --machine=${user}@.host --now --user ${service_name}",
     creates => "${xdg_config_home}/systemd/user/default.target.wants/mpd.service",
-    user    => $user,
     path    => ['/bin', '/usr/bin'],
   }