system-conf: drop reference to ShutdownWatchdogUsec=
Commit 65224c1d renamed ShutdownWatchdogUsec
into RebootWatchdogUsec but left a reference of ShutdownWatchdogUsec in
system.conf.
(cherry picked from commit 45b1fc3a)
(cherry picked from commit 54b5a1b2)
(cherry picked from commit d582e0ca)