mailbox: mediatek: remove implementation related to atomic_exec

After implement flush, client can flush the executing
command buffer or abort the still waiting for event
command buffer, so controller do not need to implement
atomic_exe feature. remove it.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>