gitlab-shell: Remove vendored files

GitLab-Shell used to implement the Git hooks that performed authorization and authentication. These were ported to Go, and since 61dcdf96 these are merged, meaning that the code this change removes isn't ever called. As such, we should remove it.

Removing these files required updates to the Makefile as some make targets are no longer needed as the gitlab-shell tests aren't executed anymore. Additionally note the first commit on this branch, as it refactors a test in the gitlabshell package to not require the Ruby script in the ruby/gitlab-shell directory. Now we depend on the Golang version implementation of JSON only.