wifi: ath12k: remove obsolete struct wmi_start_scan_arg

ath12k inherited struct wmi_start_scan_arg from ath11k. However, in
ath12k, this struct is unused -- struct ath12k_wmi_scan_req_arg is
used for this functionality. So remove the unused struct.

No functional changes, compile tested only.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240226-ath12k-obsolete-wmi_start_scan_arg-v1-1-07b0b563cb27@quicinc.com