scsi: rnbd: Remove WRITE_SAME support

REQ_OP_WRITE_SAME was only ever submitted by the legacy Linux zeroing code,
which has switched to use REQ_OP_WRITE_ZEROES long before rnbd was even
merged.

Link: https://lore.kernel.org/r/20220209082828.2629273-4-hch@lst.de


Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>