cpuidle: cpuidle-cps: remove unused variable

'core' in cps_cpuidle_init has never been used and is unnecessary, so
remove the dead code.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>