docs: conf.py: get rid of the now unused kerneldoc_bin env var

docs: conf.py: get rid of the now unused kerneldoc_bin env var

In the past, this contained the location of the binary file to parse
kernel-doc. Nowadays, it is used only for debugging purposes, inside
kerneldoc.py extension.

Move it to sphinx/kerneldoc.py, to avoid needing to handle with it
on several places.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <c0fa9f561c1dd4370f9096de4195a9214763c51c.1768823489.git.mchehab+huawei@kernel.org>