[FIX] MSIE8 errors out when trying to delete a property on the window host object, so set jsonp callback to null instead

bzr revid: xmo@openerp.com-20120831115759-m8hr9o1a7w2h9b4u

addons/web/static/src/js/data_import.js

@@ -18,7 +18,7 @@ function jsonp(form, attributes, callback) {
     attributes = attributes || {};
     var options = {jsonp: _.uniqueId('import_callback_')};
     window[options.jsonp] = function () {
-        delete window[options.jsonp];
+        window[options.jsonp] = null;
         callback.apply(null, arguments);
     };
     if ('data' in attributes) {