mailbox: mtk-cmdq: Remove proprietary cmdq_task_cb

rx_callback is a standard mailbox callback mechanism and could cover the
function of proprietary cmdq_task_cb, so use the standard one instead of
the proprietary one. Client driver has changed to use standard
rx_callback, so remove proprietary cmdq_task_cb.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>