ACPI: Drop device start operation that is not used

The ACPI device start operation, acpi_op_start, is never used, so
drop it.

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