Remove unused method Poll::Question.answerable_by

This method isn't used since commit 909114bcf.

app/models/poll/question.rb

@@ -43,12 +43,6 @@ class Poll::Question < ApplicationRecord
 
   delegate :answerable_by?, to: :poll
 
-  def self.answerable_by(user)
-    return none if user.nil? || user.unverified?
-
-    where(poll_id: Poll.answerable_by(user).pluck(:id))
-  end
-
   def options_total_votes
     question_options.reduce(0) { |total, question_option| total + question_option.total_votes }
   end