cve-check: remove deprecated CVE_CHECK_CVE_WHITELIST

This variable has been deprecated since Yocto Project version 3.0.

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit f8ac5856)
Signed-off-by: Steve Sakoman <steve@sakoman.com>