PM / Sleep: Remove unused symbol 'suspend_cpu_hotplug'

Remove the suspend_cpu_hotplug declaration, which doesn't correspond
to an existing variable.

[rjw: Added the changelog.]

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>