media: v4l2-mediabus: Drop legacy V4L2_MBUS_CSI2_CHANNEL_* flags

The V4L2_MBUS_CSI2_CHANNEL_* flags are a legacy API. Only
V4L2_MBUS_CSI2_CHANNEL_0 is used, set in a single driver, and never
read. Drop those flags. Virtual channel information should be conveyed
through frame descriptors instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>