media: ivtv/cx18: remove ivtvdriver.org references

That URL is no longer valid, so either remove references to it or
replace it with linuxtv.org.

Rather than updating the URL I've just dropped the cx18.rst driver
documentation since it was really out of date.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-by:  <awalls@md.metrocast.net>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>