Remove ChefDK’s Omnitruck JSON

Now that https://github.com/opscode-cookbooks/opscode-ci/pull/149 has 
been merged we no longer need to maintain build -> test platform 
mappings in multiple locations. The pipeline’s build to test matrix is 
now the single source of truth. Once the Chef pipelines have been fully
migrated over to the Manhattan cluster in new CI we can also remove: 
`jenkins/release.rb`.
Merge pull request #189 from opscode/schisamo/remove-chefdk-omnitruck-assets

Remove ChefDK’s Omnitruck JSON
Override the chef version for release.