media: videodev2.h: drop V4L2_CAP_ASYNCIO

The V4L2_CAP_ASYNCIO capability was never implemented (and in fact
it isn't clear what it was supposed to do in the first place).

Drop it from the capabilities list. Keep it in videodev2.h with the
other defines under ifndef __KERNEL__ for backwards compatibility.

This will free up a capability bit for other future uses. And having
an unused and undefined I/O method is just plain confusing.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>