wifi: ath12k: avoid duplicated vdev down

ath12k_mac_op_unassign_vif_chanctx() will do vdev down in function
ath12k_mac_monitor_stop(). So no need to do vdev down again.

remove the duplicated one.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3

Signed-off-by: Kang Yang <quic_kangyang@quicinc.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240429081525.983-4-quic_kangyang@quicinc.com