Remove legacy delayed job code and table

Now that the sidekiq-powered jobs have been deployed and the remaining
delayed jobs have been worked off, the delayed job code can be
removed and the table dropped.
Only skip railties when compiling asssets

So it turns out active_record itself was always being loaded, because
delayed_job_activerecord was requiring it. Now that that's gone, we
need to explicitly require it here, otherwise the isbn_validation gem
falls over. This also gets rid of the reference to
DISABLE_ACTIVE_RECORD, as that doesn't appear to be in use anymore.
Merge pull request #1405 from alphagov/cleanup-delayed-job

Remove legacy delayed job code and table