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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

scripts/sphinx-pre-install

···
 	}
 
 	$cur_version = get_sphinx_version($sphinx);
-	die ("$sphinx returned an error") if (!$cur_version);
-
 	die "$sphinx didn't return its version" if (!$cur_version);
 
 	if ($cur_version lt $min_version) {