Drop lts-alt-cve-triage.sh script

The script was added as workaround for a security-tracker problem which
was tracked as https://bugs.debian.org/919977 where the security-tracker
behind CDN returned stale data for the json export.

After some changes on the infrastructure done by DSA the problem seem to
have gone. As such we do not need two scripts for the same thing as the
alternative script really was only added as workaround to keep LTS folks
having a working CVE triage script.