ath10k: remove futile fragmentation threshold config

Commit 1010ba4c ("ath10k: unregister and
remove frag_threshold callback") didn't remove all
instances of (futile) fragmentation threshold
configuration. No known firmware supports the
parameter so don't even bother setting it.

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