video: Drop references to CONFIG_VIDEO et al

Drop the Kconfigs which are not used and all references to them. In
particular, this drops CONFIG_VIDEO to avoid confusion and allow us to
eventually rename CONFIG_DM_VIDEO to CONFIG_VIDEO.

Also drop the prototype for video_get_info_str() which is no-longer used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Jason Liu <jason.hui.liu@nxp.com>