ARM: CPU hotplug: remove bug checks in platform_cpu_die()

platform_cpu_die() is entered from the CPU's own idle thread, which
can not be migrated to other CPUs.  Moreover, the 'cpu' argument
comes from the thread info, which will always be the 'current'
CPU.  So remove this useless bug check.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>