docs: get rid of kernel-doc-nano-HOWTO.txt

Everything there is already described at
Documentation/doc-guide/kernel-doc.rst. So, there's no reason why
to keep it anymore.

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