ACPI / documentation: Remove reference to acpi_platform_device_ids from enumeration.txt

As of:

    48459340 ACPI / scan: use platform bus type by default for _HID enumeration

ACPI uses the platform bus by default, changing the opt-in to an opt-out
policy, eliminating the acpi_platform_device_ids table and replacing it
with forbidden_id_list[].

Remove the qualifying paragraph from the acpi/enumeration documentation
as it no longer applies.

Reported-by: Max Eliaser <max@meliaserlow.dyndns.tv>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>