PM: sleep: Remove obsolete comment from unlock_system_sleep()

With the freezer changes introduced by commit f5d39b02
("freezer,sched: Rewrite core freezer logic"), the comment in
unlock_system_sleep() has become obsolete, there is no need to
retain it.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>