ath6kl: remove incorrect reset_resume handler

Existing implementation of reset_resume handler just calls
ath6kl_usb_remove() that deallocates all resources.
It can lead to double free, etc. on disconnect.

The patch removes reset_resume handler,
so usb core could conservatively reset the driver.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>