ath10k: remove extra_tx_headroom

Comment was out-of-date. The headroom is no longer
necessary because HTT Tx fragment list is stored
in dma pool item associated with each sk_buff.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>