Documentation/kernel-docs.txt: get rid of broken docs

There are still some broken docs: the URLs point to somewhere,
however, the texts are not there anymore. I was able to
find the texts on other URLs for some of those, but they're all
too old. So, just get rid of them.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>