cve-update: remove cleanup of db_file in downloads

cve-update: remove cleanup of db_file in downloads

Since the code was changed to update the DB in temporary file, code
cleaning the final file in downloads is never executed.
Remove it.

Since the code always removes both files in temporary directory, remove
also comment which is trying to differentiate this code from code just
removed.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>