target/iscsi: Define OFFLOAD_BUF_SIZE once

The constant OFFLOAD_BUF_SIZE is defined twice - once in
iscsi_target_seq_pdu_list.c and once in iscsi_target_erl1.c. Since
that constant is not used in the former source file, remove its
definition from that source file.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Mike Christie <mchristi@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>