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`.

Gemfile.lock

@@ -135,11 +135,6 @@ GEM
     dead_end (3.1.0)
     debug_inspector (1.1.0)
     declarative (0.0.20)
-    delayed_job (4.1.9)
-      activesupport (>= 3.0, < 6.2)
-    delayed_job_active_record (4.1.6)
-      activerecord (>= 3.0, < 6.2)
-      delayed_job (>= 3.0, < 5)
     derailed_benchmarks (2.1.1)
       benchmark-ips (~> 2)
       dead_end
@@ -537,8 +532,6 @@ DEPENDENCIES
   clockwork
   codecov
   daemons
-  delayed_job
-  delayed_job_active_record
   derailed_benchmarks
   diff-lcs
   dotenv-rails