freezer: remove obsolete comments in __thaw_task()

__thaw_task() no longer clears frozen flag since commit a3201227
(freezer: make freezing() test freeze conditions in effect instead of TIF_FREEZE).

Reviewed-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>