target/iscsi: Remove dead code from iscsit_process_scsi_cmd()

If an iSCSI command is rejected before iscsit_process_scsi_cmd()
is called, .reject_reason is set but iscsit_process_scsi_cmd() is
not called. This means that the "if (cmd->reject_reason) ..." code
in this function can be removed without changing the behavior of
this function.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andy Grover <agrover@redhat.com>
Cc: David Disseldorp <ddiss@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>