ath9k: Remove unused IEEE80211_WEP_NKID

IEEE80211_WEP_NKID is not used in ath9k any more since the key handling code
has been moved to ath/.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>