core: remove unused code, replace queue flash

Remove unused THROTTLE code. Replace standard flush_scheduled_work() with
work queue specific flush_workqueue() method.

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>
rtc: declare function static

Declare funciion get_rtc_time(...) as static. The function must not be public.

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>