Widgets: Remove `WP_Widget_Factory::$hashed_class_counts` property, unused since [46220].

The `spl_object_hash()` function was introduced in PHP 5.2.0. As of PHP 5.3, the PHP SPL extension can no longer be disabled, so the `WP_Widget_Factory::hash_object()` workaround was removed in [46220].

See #48074.
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