Remove legacy code from ProcessPipelineService
Processing pipelines generates a lot of events, as we have legacy code to make sure that we process legacy pipelines, something that we should remove, as this generates congestion as it seems that we do O(n^2) operation, where n is number of jobs.
Given we do not have a way to not enqueue duplicates, due to how we push data to sidekiq, it makes the queues grow rapidly when processing big pipelines.
We should remove this code https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/services/ci/process_pipeline_service.rb#L82 already marked for deprecation.
(adapted from @ayufan's soliloquy here https://gitlab.slack.com/archives/C101F3796/p1490386687980475)