Remove config.threadsafe! from production env template

Closes #8232 [ci skip]

railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt

@@ -65,9 +65,6 @@
   # Set this to true and configure the email server for immediate delivery to raise delivery errors.
   # config.action_mailer.raise_delivery_errors = false
 
-  # Enable threaded mode.
-  # config.threadsafe!
-
   # Enable locale fallbacks for I18n (makes lookups for any locale fall back to
   # the I18n.default_locale when a translation can not be found).
   config.i18n.fallbacks = true