ath6kl: Remove unnecessary node table update on disconnect event

Since ath6kl does not actually update cfg80211 BSS table when this
event occurs, there is not much need for removing the entries from
the internal table that is not really used or exposed.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>