core/service: downgrade "scheduling restart" message to debug

I see we log this during every boot, even though it is a routine expected event:
Nov 12 14:50:01 krowka systemd[1]: systemd-journald.service: Service has no hold-off time (RestartSec=0), scheduling restart.
(and for other services too). Let's downgrade this to debug level.

https://bugzilla.redhat.com/show_bug.cgi?id=1614871
(cherry picked from commit 868f7d36)