IB/iser: Remove deprecated pi_guard module param

No need for this dead code. This commit doesn't change any functionality
since one can still run "modprobe ib_iser pi_guard=<type>".

Link: https://lore.kernel.org/r/20211215135721.3662-2-mgurtovoy@nvidia.com


Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>