EntityAPIController::delete() doesn't use a database transaction

In #1027908-47: entity_load() should return entities keyed by ID, not name, EntityAPIController::delete() had some logic for its $transaction parameter commented out. Seeing as the blocking issue has been fixed for about 18 months now, it might be time to re-enable that code :)

Without the change, entity deletions will not use a transaction at all unless you override the method and provide one yourself.