ACPI: Drop removal_type field from struct acpi_device

The ACPI processor driver was the only user of the removal_type
field in struct acpi_device, but it doesn't use that field any more
after recent changes.  Thus, removal_type has no more users, so drop
it along with the associated data type.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Toshi Kani <toshi.kani@hp.com>