Enables the host to remove its menus and toolbars.
Returns S_OK if successful, or an error value otherwise.
If a host displays menus and toolbars during the call to IDocHostUIHandler::ShowUI, the host should remove them when this method is called. This method is called regardless of the return value from the IDocHostUIHandler::ShowUI method.
To avoid a memory leak, forward calls to IDocHostUIHandler::HideUI to the default UI handler.