Clean up systemd units

Use Type=oneshot for the service and drop the default Restart=no.
Remove Persistent=true from the timer, which only applies to
OnCalendar= timers.

update-notify.service

@@ -3,5 +3,4 @@ Description=Notify if updates are available
 
 [Service]
 ExecStart=/usr/bin/update-notify 30
-Type=simple
-Restart=no
+Type=oneshot

update-notify.timer

@@ -4,7 +4,6 @@ Description=Run update-notify every 6 hours
 [Timer]
 OnBootSec=5min
 OnUnitActiveSec=6h
-Persistent=true
 
 [Install]
 WantedBy=timers.target