docs-rst: don't require adjustbox anymore

Only the media PDF book was requiring adjustbox, in order to
scale big tables. That worked pretty good with Sphinx versions
1.4 and 1.5, but Spinx 1.6 changed the way tables are produced,
by introducing some weird macros before tabulary.
That causes adjustbox to fail. So, it can't be used anymore,
and its usage was removed from the media book.

So, let's remove it from conf.py and sphinx-pre-install.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>