iscsi/iser-target: fix build warning

With upstream commit defd8848, 3.12
commit 2f82fa98, we introduced a
warning:
  drivers/infiniband/ulp/isert/ib_isert.c: In function 'isert_cq_rx_comp_err':
  drivers/infiniband/ulp/isert/ib_isert.c:##L##: warning: unused variable 'ib_dev' [-Wunused-variable]

Remove ib_dev as it is unused.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/infiniband/ulp/isert/ib_isert.c

@@ -1652,7 +1652,6 @@ isert_cq_tx_comp_err(struct iser_tx_desc *tx_desc, struct isert_conn *isert_conn
 static void
 isert_cq_rx_comp_err(struct isert_conn *isert_conn)
 {
-	struct ib_device *ib_dev = isert_conn->conn_cm_id->device;
 	struct iscsi_conn *conn = isert_conn->conn;
 
 	if (isert_conn->post_recv_buf_count)