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.]

Change-Id: Ie09bdd868622c7d5d0304f68f9b9f9b784ca42ea
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>