Remove old code for standalone CiviCRM

Remove \CRM_Core_Invoke::hackStandalone - and the single call to it. It refers to a removed function (CRM_Core_Standalone::).

This path/handle no longer exist and should be removed from the schema XML and DB?

civicrm/standalone/register CRM_Standalone_Form_Register

There's also a comment reference in \CRM\Core\Session.php

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