ActivityManager : Remove POWER_OFF_ALARM_MODE HOME_KEY button override

This is being removed for 2 reasons :
1) The feature sometimes enters in an inconsistent state where
user is stuck in this mode even when its unintential. This is
addressed by http://review.cyanogenmod.org/#/c/133956/

2) There is a security concern regarding this being in System table.
This means any third party app can toggle this setting and keep
the user from exiting their app.

Change-Id: I36393bda81e087264629d801b4bdf62b11837187
CYNGNOS-2119
(cherry picked from commit 1291c93f)