LU-9480 lnet: refactor lnet_del_peer_ni()

LU-9480 lnet: refactor lnet_del_peer_ni()

Refactor lnet_del_peer_ni(). In particular break out the code
that removes an lnet_peer_ni from an lnet_peer and put it into
a separate function, lnet_peer_del_nid().

Test-Parameters: trivial
Signed-off-by: Olaf Weber <olaf@sgi.com>
Change-Id: Id5988b308afb093f83cc2e7029d3f2961171c906
Reviewed-on: https://review.whamcloud.com/25779
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Reviewed-by: Amir Shehata <amir.shehata@intel.com>
Tested-by: Amir Shehata <amir.shehata@intel.com>