scripts/sphinx-pre-install: don't use LaTeX with CentOS 7

[ Upstream commit 56e5a633 ]

There aren't enough texlive packages for LaTeX-based builds
to work on CentOS/RHEL <= 7.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>