DEV: Clean up old polls data from custom fields

In early 2015, the poll plugin was writing its data to custom fields on
the post containing the poll. It was later changed to have dedicated SQL
tables and the polls were migrated but we forgot to clean the existing
data.

plugins/poll/db/post_migrate/20230612134421_remove_old_polls_data_from_custom_fields.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class RemoveOldPollsDataFromCustomFields < ActiveRecord::Migration[7.0]
+  def up
+    execute <<~SQL
+    DELETE FROM post_custom_fields
+    WHERE name LIKE 'polls%'
+    SQL
+  end
+
+  def down
+  end
+end