[media] Documentation: README.ivtv: Remove note that ivtvfb is not yet in the kernel

ivtvfb is now in the kernel, so stop saying it's not.

Signed-off-by: Paul Cassella <fortytwo-ivtv@manetheren.bigw.org>
Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>