ACPI: Drop redundant acpi_dev_parent() header

Because acpi_dev_parent() is defined as static inline, the extra
header of it in acpi_bus.h is redundant, so drop it.

Fixes: 62fcb99b ("ACPI: Drop parent field from struct acpi_device")
Reported-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>