ACPI / processor: Remove outdated comments

acpi_get_processor_id() can be find nowhere, and the acpi id
is synchronized to APIC id when acpi_get_cpuid() is called, so
the comments can be removed.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>