MAINTAINERS: Drop stale F: from "Sphinx documentation configuration ..."

We neglected to update MAINTAINERS when we fused the conf.py into one,
and when we deleted the kernel-doc script.  Update it now.

Fixes: b93f4fbdc4 (docs: Build and install all the docs in a single manual, 2021-01-15)
Fixes: 619d5f0211 (scripts/kernel-doc: Delete the old Perl kernel-doc script, 2025-08-14)
Cc: John Snow <jsnow@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20260603064032.3733394-38-armbru@redhat.com>