wifi: ath9k: btcoex: remove platform_data

This is completely unused as platform files are no longer used anywhere.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://patch.msgid.link/20240906195359.6982-3-rosenp@gmail.com