Fix the `Gitlab::Seeder` monkey patch to disable mail delivery
At some point our Notify.deliver_later monkey patch stopped working, so during the dev:setup task we were actually processing emails for the relevant records that we created, including about 2,500 just for comments. Each email resulted in a file being written to disk for Letter Opener.
| Before | bundle exec rake dev:setup 1583.14s user 121.58s system 96% cpu 29:30.86 total |
| After | bundle exec rake dev:setup 295.01s user 17.87s system 90% cpu 5:47.06 total |
Note that the "After" includes two additional seeds not present for the "Before" run, 18_abuse_reports and 19_nested_groups, the latter of which isn't exactly speedy, so the difference is even more substantial.