Remove Debian 8 builds as it's been EOL'd upstream
In addition, Chef no longer creates builds for this in 16 [1] and 15 [2].
[1] https://discourse.chef.io/t/chef-infra-client-16-2-released/17284 [2] https://discourse.chef.io/t/chef-infra-client-15-12-22-released/17316