Simply deleting a plugin like that, e.g.
<!--[if lt IE 8]><!-->
<script>
delete $.fn.foundation;
</script>
<!--<![endif]-->
Will probably cause all sorts of problems and is generally a bad idea - see here for why.
Instead, just load the foundation()
plugin/related scripts if the browser is IE8+ or a different browser:
<!--[if gte IE 8]><!-->
<script src="/js/foundation.min.js"></script>
// Other related scripts etc...
<!--<![endif]-->