patchtest: remove test for CVE tag in mbox

After patchtest went live it was determined that testing for a CVE tag
in the mbox commit message is unnecessary, since it will already be in
the shortlog and in any carried patches. Remove the test and the
associated selftest files so that its absence isn't flagged in future
test results.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>