ath11k: remove unused ATH11K_BD_IE_BOARD_EXT

Currently ATH11K_BD_IE_BOARD_EXT is not used, so remove it.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03125-QCAHSPSWPL_V1_V2_SILICONZ_LITE-3

Signed-off-by: Wen Gong <quic_wgong@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220319023543.14288-2-quic_wgong@quicinc.com