scsi: remove useless acpi functions in the header file

commit f1bc1e4c ("ata: acpi: rework the ata acpi bind support")
removed scsi_register_acpi_bus_type() and
scsi_unregister_acpi_bus_type(), but forgot to remove them in the header
file, do it now.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Reviewed-by: John Garry <john.garry@huawei.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>