media: v4l2-async: Remove V4L2_ASYNC_MATCH_CUSTOM

Custom/driver-specific v4l2-async match support was introduced
in 2013, as V4L2_ASYNC_BUS_CUSTOM.

This type of match never had any user, so it's fair
to conclude it's not required and that safe for removal.
If the support is ever needed, it can always be restored.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>