media: soc_camera: Remove leftover files, add TODO

Remove a few files left over from the mt9t031 driver. While at it, add a
TODO file for the SoC camera framework as a whole.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>