Drop sentry async - use Sentry's builtin background worker

See getsentry/sentry-ruby#1522 for details and
why we want to do this change.

We suspect this also may be a cause of https://app.shortcut.com/simpledotorg/story/5282/redis-down-in-production

config/initializers/sentry.rb

@@ -1,5 +1,2 @@
 Sentry.init do |config|
- config.async = lambda do |event, hint|
- Sentry::SendEventJob.perform_later(event, hint)
- end
 end