net80211: remove ieee80211_unref_node()

ieee80211_unref_node() was only used in two error cases in
ieee80211_send_nulldata(). There we do not need to guard against
ni pointer reuse after decrementing the refcount of the ni as we
only update the stats and return.

Update the man page and remove the link for the now gone function.

Note: the last uses of the function were removed in 493d6255434ce.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian, emaste
Differential Revision: https://reviews.freebsd.org/D42035

(cherry picked from commit 1137943b1c5b225e30b82350ff7fb12c258ea90f)