wifi: ath12k: fix conf_mutex in ath12k_mac_op_unassign_vif_chanctx()

"mutex_unlock(&ar->conf_mutex);" is always called at end of function
ath12k_mac_op_unassign_vif_chanctx(), so delete this unlock to make
sure lock/unlock is paired.

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: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230608104444.3134-1-quic_wgong@quicinc.com