docs: remove cdomain.py

This is not used anymore, as it was a logic we had to support
pre Sphinx 3.x, as shown at:

    afde706 ("Make the docs build "work" with Sphinx 3.x")

Remove it.

Fixes: b267178 ("docs: conf.py: drop backward support for old Sphinx versions")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <3b86d236c64af17924e4cfedbbfb8bc60059802f.1758381727.git.mchehab+huawei@kernel.org>

Author: mchehab