ath10k: remove set/get_tsf ieee80211_ops

Neither of these did the right thing:

- get_tsf just returned 0
- set_tsf assumed a simple offset was applied against
  get_tsf(), which works, except in the case of
  calculating TSF from rx_mactime (actual TSF).

Just remove them for now. We can reimplement set_tsf in
terms of TSF increment/decrement in the future if get_tsf
is ever supported by FW.

Signed-off-by: Thomas Pedersen <twp@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>