ath9k: remove ath9k_sta_remove_debugfs

mac80211 uses debugfs_remove_recursive, so there's no need for the
driver to do an explicit cleanup of its sta debugfs entry.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>