Revert "test: vul-webapp failed in servlet 2.3"

This reverts commit 93971fc66c.

vul/vul-webapp/src/main/webapp/WEB-INF/web.xml

@@ -1,5 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<web-app>
+<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee"
+         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
+         id="WebApp_ID" version="2.5">
     <welcome-file-list>
         <welcome-file>index.html</welcome-file>
     </welcome-file-list>
@@ -101,9 +103,9 @@
         <servlet-name>biginteger</servlet-name>
     </filter-mapping>
 
-<!--    <listener>-->
-<!--        <listener-class>EmptyListener</listener-class>-->
-<!--    </listener>-->
+    <listener>
+        <listener-class>EmptyListener</listener-class>
+    </listener>
     <filter>
         <filter-name>emptyFilter</filter-name>
         <filter-class>EmptyFilter</filter-class>