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 actual 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: Ic545191713c607d4e186dfdd05949a775b45eadc
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>