docs: Makefile: get rid of KERNELDOC_CONF env variable

docs: Makefile: get rid of KERNELDOC_CONF env variable

Despite its name, what's there is a set of Sphinx arguments that
are passed to sphinx/kerneldoc.py:

- kerneldoc_srctree: location of the source tree;
- kerneldoc_bin: external script to excecute kernel-doc

Drop it, and just place the values at the already-existing
ALLSPHINXOPTS variable.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <88be0fc288598c30fbedb9cc230b2a7ed28225a2.1745453655.git.mchehab+huawei@kernel.org>