ACPI: Drop acpi_evaluate_hotplug_ost() and ACPI_HOTPLUG_OST

Replace acpi_evaluate_hotplug_ost() with acpi_evaluate_ost()
everywhere and drop the ACPI_HOTPLUG_OST symbol so that hotplug
_OST is supported unconditionally.

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