scripts: sphinx-pre-install: remove unnecessary double check for $cur_version

scripts: sphinx-pre-install: remove unnecessary double check for $cur_version

$cur_version is currently being tested twice with the first test
resulting in an unhelpful "$sphinx returned an error", not continuing to
the more helpful "$sphinx didn't return its version".

This patch removes the first test to return the more useful message.

Fixes: a8b380c379ef ("scripts: sphinx-pre-install: only ask to activate valid venvs")
Signed-off-by: Sebastian Muxel <sebastian@muxel.dev>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240827133224.160776-1-sebastian@muxel.dev