oomd: "downgrade" level of message

PID1 already logs about the service being started, so this line isn't necessary
in normal use. Also, by the time it is emitted, the service has already
signalled readiness, so let's not say "starting" but "started".

src/oom/oomd.c

@@ -170,7 +170,7 @@ static int run(int argc, char *argv[]) {
 
         notify_msg = notify_start(NOTIFY_READY, NOTIFY_STOPPING);
 
-        log_info("systemd-oomd starting%s!", arg_dry_run ? " in dry run mode" : "");
+        log_debug("systemd-oomd started%s.", arg_dry_run ? " in dry run mode" : "");
 
         r = sd_event_loop(m->event);
         if (r < 0)