Stop using sidekiq

[jcoyne]

Because we never use it to run any jobs
Fixes #449

See log messages indicating we are just using the async adapter:

I, [2023-04-11T09:15:02.013819 #1507467]  INFO -- : [ActiveJob] [GarbageCollectJob] [16d7b9c3-c24b-4690-90e8-1c25d12adf89] Performing GarbageCollectJob (Job ID: 16d7b9c3-c24b-4690-90e8-1c25d12adf89) from Async(default) enqueued at

contentsearch@sul-contentsearch-prod-a:~/contentsearch/current$ bin/rails c -e p
Loading production environment (Rails 7.0.4.3)
3.1.0 :001 > Rails.application.config.active_job.queue_adapter
 => :async