DEV: remove experimental lightbox site setting (#36377)

Follow up to #36375 to remove the redundant site setting from the
database.

db/migrate/20251202101647_remove_experimental_lightbox_site_setting.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class RemoveExperimentalLightboxSiteSetting < ActiveRecord::Migration[8.0]
+  def up
+    execute "DELETE FROM site_settings WHERE name='experimental_lightbox'"
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end