wifi: ath9k: Remove unused declarations

Commit b57ba3b2 ("ath9k: move ath9k_reload_chainmask_settings to common")
removed the implementation ath9k_reload_chainmask_settings() but not the declaration.
Also commit 978f78bf

 ("ath9k_hw: Move bt_stomp to hw from common.") left behind
ath9k_cmn_btcoex_bt_stomp() declaration.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230810145222.34364-1-yuehaibing@huawei.com