Remove 3rd argument to normalize_wwn

possible_wwns is no longer used by any callers.

Signed-off-by: Andy Grover <agrover@redhat.com>
Use to_normalized_wwn in _set_nexus

We can now use to_normalized_wwn, since it no longer checks against
wwn_list.

Signed-off-by: Andy Grover <agrover@redhat.com>