iser-target: Remove explicit mlx4 work-around

The driver now exposes sufficient limits so we can
avoid having mlx4 specific work-around.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>