wifi: ath12k: remove unused ATH12K_BD_IE_BOARD_EXT

Currently ATH12K_BD_IE_BOARD_EXT is not used, so remove it.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0-03427-QCAHMTSWPL_V1.0_V2.0_SILICONZ-1.15378.4

Signed-off-by: Wen Gong <quic_wgong@quicinc.com>
Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20231216060140.30611-4-quic_bqiang@quicinc.com