ACPI: processor: Remove the no longer needed __weak arch_register_cpu()

acpi_processor.c has a weak definition of arch_register_cpu() for arm64,
as arm64 didn't previously define one. Now that it does, remove it.

This will let ia64 and x86 fall-back on the common code definition
when CONFIG_HOTPLUG_CPU is not defined.

Signed-off-by: James Morse <james.morse@arm.com>