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`.

db/migrate/20220104214319_drop_delayed_jobs.rb

@@ -0,0 +1,24 @@
+class DropDelayedJobs < ActiveRecord::Migration[6.1]
+  def up
+    drop_table :delayed_jobs
+  end
+
+  def down
+    create_table :delayed_jobs do |t|
+      t.integer  :priority, default: 0
+      t.integer  :attempts, default: 0
+      t.text     :handler
+      t.text     :last_error
+      t.datetime :run_at
+      t.datetime :locked_at
+      t.datetime :failed_at
+      t.string   :locked_by
+      t.timestamps null: false
+      t.string   :queue
+    end
+
+    add_index :delayed_jobs, :run_at
+    add_index :delayed_jobs, :locked_at
+    add_index :delayed_jobs, :locked_by
+  end
+end