wifi: ath12k: remove unused variable monitor_flags

monitor_flags is defined in struct ath12k. Although it is changed in
some places, but it is not actually used.

So remove related code.

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/20240429081525.983-2-quic_kangyang@quicinc.com