docs: conf.py: get rid of load_config.py

docs: conf.py: get rid of load_config.py

The code here was meant to handle 3 functions:
1. allow having a separate conf.py file, per subdir;
2. generate a list of latex documents.
3. set "subproject" tag if SPHINXDIRS points to a subdir.

We don't have (1) anymore, and (3) is now properly handled
entirely inside conf.py.

So, only (3) is still needed, and this is a single-line change
at conf.py.

So, drop it, moving the remaining code to conf.py.

While here, drop a duplicated $(RUSTDOC) command-line argument.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <ec998f9f268a401ca6aa36e3221d39c97efeccaa.1758361087.git.mchehab+huawei@kernel.org>