rtc:alarm: remove alarm_shutdown hook in alarm driver

Putting alarm_shutdown into set_power_on_alarm is to set rtc alarm
time immediately after an alarm being set. It can avoid shutting
down unexpectedly before calling alarm_shutdown hook like removing
battery.Then power_on_alarm will be set to rtc twice when an alarm
is created and the device performs shutdown for a power off alarm.
The set process during shutting down is duplicated to the set
process when an alarm is created. So the alarm_shutdown hook become
unnecessary.

CRs-Fixed: 639119
Change-Id: Ie46bcf6f568f04650d94ba40ad6d4521d31a2f96
Signed-off-by: Xiaocheng Li <lix@codeaurora.org>
2 files changed