ath9k: Remove 'stations' debugfs file

The 'stations' debugfs file has multiple issues. It doesn't scale
to an arbitrary number of associated stations and allocating
64K is not elegant either. Now that changes have been made in
mac80211 to support dynamic creation/deletion of driver-specific
debugfs files on station addition/removal, remove this file and
make use of the mac80211 hooks (which will be done in a sebsequent
patch).

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