Remove unused gitlab-shell/Gemfile
We vendored gitlab-shell to get its hooks. In doing so, we vendored its Gemfile.
Our tests stopped using that Gemfile a while ago already; make rspec-gitlab-shell
uses ruby/Gemfile
. Also gitlab-shell does not use gems in production. So we should just delete that Gemfile.
This came to our attention because the dependency scanner does list the gems in gitlab-shell.