wifi: ath11k: Remove unused struct ath11k_htc_frame

struct ath11k_htc_frame is unused, and since it illogically contains
two consecutive flexible arrays, it could never be used, so remove it.

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://lore.kernel.org/r/20231009-ath11k_htc_frame-v1-1-81d405b7a195@quicinc.com