FIX: no error when double save post with poll (#13111)

Some plugins hook into Post after save to set custom fields and save again.

For example: https://github.com/discourse/discourse-category-experts/blob/main/lib/category_experts/post_handler.rb#L27

Problem is that in case like that `raw_changed?` is false but all callback are triggered. `extracted_polls` is class atribute therefore that should be reset with each attempt.

That was causing an error:
```
#<ActiveRecord::RecordNotUnique: PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint 
"index_polls_on_post_id_and_name" DETAIL:  Key (post_id, name)=(8967, poll) already exists.
```

plugins/poll/plugin.rb

@@ -467,6 +467,7 @@ after_initialize do
 
       after_save do
         polls = self.extracted_polls
+        self.extracted_polls = nil
         next if polls.blank? || !polls.is_a?(Hash)
         post = self