Remove sidekiq worker

Story: https://www.pivotaltracker.com/story/show/82200576

Since the Email Alert API now queues received notifications from the Email Alert Service, Sidekiq is no longer needed.
The answer is no, sidekiq does it: https://github.com/mperham/sidekiq/blob/27fc88504b15ea5b57929845c7118de66c3120ea/lib/sidekiq/redis_connection.rb#L32-L37