vex: drop obsolete conflict check with cve-check class

vex: drop obsolete conflict check with cve-check class

cve-check class was removed, so this check is obsolete.

>From commit message introducing vex class it's not clear why this check
is present, but presumably to not extend the cve status multiple times.
Thas is no longer possible since commit
45e18f4270d084d81c21b1e5a4a601ce975d8a77 introduced sanity check to
prevent that from happening.
Therefore it's also not necessary to rewrite this check to new
sbom-cve-check(-recipe) classes.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>