[media] omap3isp: preview: Remove horizontal averager support

The horizontal averager isn't used and will get in the way when
implementing cropping support on the input pad. Remove it, it can be
added back later if needed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>