core_ctl: kill maintenance of online state

The online state is cached in CPU hotplug callbacks. This is
used in determining the potential candidates for isolation or
unisolation. The current code has checks in hotplug notifier
callback to correct the online state if it is out of sync
with the actualy CPU state. All this maintenance can be
avoided by directly using cpu_online().

This patch fixes an incorrect warning message emitted from
CPU_UP_CANCELED event callback.

Change-Id: I9681ce6c5bc701507427d60db397ca4966fbdb58
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>