docs: conf.py: drop xindy rule

docs: conf.py: drop xindy rule

The rule as-is is wrong, as it was inverted. Besides that, after
retest building all repos with suggested LaTeX packages given
by sphinx-pre-install, I was unable to reproduce the issues
I saw with xindy in the past.

So, let's just drop. If anyone reports issues with xindy, we
may need to readd, but at the right way, e.g. {options}{pkgname}.

Reported-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/83068bc31839e7095f1f408e49658362d467797e.1756123459.git.mchehab+huawei@kernel.org