CorrelationId class is apparently duplicated

In GitLab CE (and EE) we have a Gitlab::CorrelationId class. This is apparently a duplication of the file from the gitlab-labkit gem, which we have in our Gemfile.

Apparently we only use the Gitlab::CorrelationId in one place:

gitlab-ce master % g grep Gitlab::Correlation
lib/gitlab/tracing.rb:33:        .gsub(/\{\{\s*correlation_id\s*\}\}/, Gitlab::CorrelationId.current_id.to_s)
spec/lib/gitlab/tracing_spec.rb:62:        allow(Gitlab::CorrelationId).to receive(:current_id).and_return(correlation_id)

cc @andrewn @ayufan