Drop halt_callback_chains_on_return_false

ActiveSupport.halt_callback_chains_on_return_false= is deprecated and
will be removed in Rails 5.2. Stop using it and return `throw(:abort)`
instead in the callbacks which need it.

app/models/conference.rb

@@ -1120,7 +1120,6 @@ class Conference < ApplicationRecord
   #
   def create_email_settings
     build_email_settings
-    true
   end
 
   ##