iscsi-target: Make iscsi_tpg_np driver show/store use generic code

Go ahead and fold the duplicate iscsi_tpg_np driver attribute
show/store functions into generic callers, invoked by current
driver specific functions.

This allows for future v4.8+ changes to have iscsi_target_mod
use tpg_np driver attributes at runtime, instead of having
the defaults hardcoded in iscsi_target_configfs.c.

Also, drop the unused/legacy 'sctp' attribute for non standard
RFC-3720 operation with IPPROTO_SCTP.

Cc: Varun Prakash <varun@chelsio.com>
Cc: Hariprasad Shenai <hariprasad@chelsio.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>