login: do not clear wall message before shutting down

This fixes a regression caused by 5ed73478e1
and aa6123e85e.

With these commits, when Reboot dbus method or friends is called,
reset_scheduled_shutdown() is called before shutting down the system,
so the wall message was cleared.

As the wall message itself is unrelated to the scheduling of shutting
down systems, let's keep the message.

Fixes #30167.

src/login/logind-dbus.c

@@ -2330,7 +2330,6 @@ static void reset_scheduled_shutdown(Manager *m) {
         m->scheduled_shutdown_timeout = USEC_INFINITY;
         m->scheduled_shutdown_uid = UID_INVALID;
         m->scheduled_shutdown_tty = mfree(m->scheduled_shutdown_tty);
-        m->wall_message = mfree(m->wall_message);
         m->shutdown_dry_run = false;
 
         if (m->unlink_nologin) {