jobs: remove DelayedJobs.

Remove the DelayedJobs gem and database table. Completes the transition
to GoodJob started in c06bfa64f and f4953549a.

Downstream users can upgrade as follows:

* Stop the Rails server.
* Stop the DelayedJobs worker (normally running as `bin/delayed_job` or `bin/rails jobs:work`).
* Run `bin/rails jobs:work` to finish any pending delayed jobs.
* Run `bin/rails db:migrate` to create the good_jobs table and drop the delayed_jobs table.
* Start the Rails server again.
* Start the GoodJobs worker with `bin/good_job start`.

bin/delayed_job

@@ -1,6 +0,0 @@
-#!/usr/bin/env ruby
-
-require File.expand_path(File.join(File.dirname(__FILE__), '..', 'config', 'environment'))
-require 'delayed/command'
-Delayed::Job.connection.execute "set statement_timeout=0"
-Delayed::Command.new(ARGV).daemonize