scripts/sphinx-pre-install: get rid of RHEL7 explicity check

RHEL8 was already launched. This test won't get it, and will
do the wrong thing. Ok, we could fix it, but now we check
Sphinx version to ensure that it matches the minimal (1.3),
so there's no need for an explicit check there.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>