scsi: target/core: Remove an unused data member from struct xcopy_pt_cmd

A value is assigned to the xcopy_op member of struct xcopy_pt_cmd but
that value is never used. Hence remove the xcopy_op member.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>