docs: LaTeX/PDF: drop list of documents

docs: LaTeX/PDF: drop list of documents

The building system can auto-generate a list of documents since
commit: dac20fbec9ef ("docs: pdf: add all Documentation/*/index.rst to PDF output").

The added logic there allows keeping the existing list, but
there's not real reason to keep it. Now, the media document
has gone (it was split into tree).

So, it sounds about time to get rid of the manual entries,
and let the script to generate it automatically instead.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9345dba7164497dbf28578f6ec271e479379610c.1586881715.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>