ACPI: memhotplug: Remove 'state' from struct acpi_memory_device

After commit 315bbae9

 ("ACPI / memhotplug: deal with eject request
in hotplug queue"), the memory device state, which is defined in
struct acpi_memory_device, is not actually useful, so remove it along
with symbols related to it.

Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>