Actually there is. You can set different logo on different page. So, you can point it to an empty image file.
// replace Roundcube logo with this image
// specify an URL relative to the document root of this Roundcube installation
// an array can be used to specify different logos for specific template files, '*' for default logo
// for example array("*" => "/images/roundcube_logo.png", "messageprint" => "/images/roundcube_logo_print.png")
$config['skin_logo'] = null;