scripts: kernel-doc: get rid of $nested parameter

The check_sections() function has a $nested parameter, meant
to identify when a nested struct is present. As we now have
a logic that handles it, get rid of such parameter.

Suggested-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>