iscsi-target: Drop left-over bogus iscsi_np->tpg_np

This patch drops the left-over iscsi_np->tpg_np pointer, now
that iser-target PI is able to dynamically allocate PI contexts
per I/O, instead of needing to determine support using a TPG
attribute with this bogus reference.

Cc: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>