media: i2c: Drop unused ad9389b video encoder driver

The ad9389b video encoder driver doesn't support DT and relies on
platform data. No board file has ever provided platform data for that
device. The driver has thus never been used in the mainline kernel since
its introduction in v3.7. Drop it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>