The debugfs code has been relocated to v3 hw driver, so delete unused
struct hisi_sas_hw function pointers snapshot_{prepare, restore}.

Link: https://lore.kernel.org/r/1617709711-195853-2-git-send-email-john.garry@huawei.comSigned-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>