FIX: couldn't delete users who voted on a poll

plugins/poll/plugin.rb

@@ -359,6 +359,10 @@ after_initialize do
     end
   end
 
+  User.class_eval do
+    has_many :poll_votes, dependent: :delete_all
+  end
+
   validate(:post, :validate_polls) do |force = nil|
     return unless self.raw_changed? || force
 
@@ -427,10 +431,6 @@ after_initialize do
     PollVote.where(user_id: source_user.id).update_all(user_id: target_user.id)
   end
 
-  on(:user_destroyed) do |user|
-    PollVote.where(user_id: user.id).delete_all
-  end
-
   register_post_custom_field_type(DiscoursePoll::HAS_POLLS, :boolean)
 
   topic_view_post_custom_fields_whitelister { [DiscoursePoll::HAS_POLLS] }