Remove containers only through build deletion request

[gvso]

Currently, there are two ways of deleting a build. The DELETE build/:build is used for manual deletion in Probo Coordinator while DELETE container/:container is used by the reaper.

Both endpoints end up calling the same function and cause overheads (build object is retrieved twice on manual deletion!). We really just need one of the endpoints.

This change depends on probo-reaper#24 (and vice-versa)

[ElusiveMind]

Re-factored for latest changes to Probo and testing via test environment.