Remove unused answer_id field in poll_voters table

This field isn't used since commit 51be80eed, right after being
added in commit 5806d86e3.

db/migrate/20240511141119_remove_answer_id_from_poll_voters.rb

@@ -0,0 +1,5 @@
+class RemoveAnswerIdFromPollVoters < ActiveRecord::Migration[7.0]
+  def change
+    remove_column :poll_voters, :answer_id, :integer
+  end
+end