target: Remove the unused flag SCF_ACK_KREF

The flag SCF_ACK_KREF is only set but never tested. Hence remove
this flag.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>