makeAjaxRequest: don't set request to null

Setting onreadystatechange can avoid the possibility of it somehow being
called twice but there's no point in nulling the request right before we
let the GC collect it.

src/openseadragon.js

@@ -1341,7 +1341,6 @@ window.OpenSeadragon = window.OpenSeadragon || function( options ){
                 $.console.log("%s while making AJAX request: %s", e.name, e.message);
 
                 request.onreadystatechange = function(){};
-                request = null;
 
                 if ($.isFunction(onError)) {
                     onError( request, e );