Remove videobook.tmpl

This document describes the long-deprecated V4L1 interface.  In-tree, it
can only serve to encourage developers to write drivers to the wrong API.
Remove it in favor of the V4L2 documentation which must surely show up
someday.

Acked-by: Alan Cox <alan@redhat.com>
Acked-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>