ath9k: Node cleanup

Start removing the internal node list in ath9k, in preparation
for using mac80211's STA list.
Remove lists, locks, routines, flags, functions managing nodes in ath9k.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>