soc: mediatek: cmdq: Remove cmdq_pkt_flush_async() helper function

cmdq_pkt_flush_async() is not used by all client drivers (MediaTek
drm driver and MediaTek mdp3 driver), so remove it.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Link: https://lore.kernel.org/r/20240222154120.16959-7-chunkuang.hu@kernel.org


Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>