docs: kdoc: remove support for an external kernel-doc from sphinx

docs: kdoc: remove support for an external kernel-doc from sphinx

The ability to build the docs with an external kernel-doc program involves
some truly confusing logic and complicates the task of moving kernel-doc
out of scripts/.  But this feature is not useful for normal documentation
builds, and the external kernel-doc can always be run by hand when it needs
debugging.  So just remove that feature and make life easier.

There is still a bunch of logic to build a command line that we never use;
the idea is to be able to output it, but I'm not sure if that is worth
keeping.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <a97a8361546648906344457a7e92e4db533048a9.1768823489.git.mchehab+huawei@kernel.org>