Fix a cppcheck warning.

Remove some dead code. Updating 'last' is pointless here.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1838271 13f79535-47bb-0310-9956-ffa450edef68
(cherry picked from commit e9d215e)

server/listen.c

@@ -370,11 +370,10 @@ static const char *set_systemd_listener(process_rec *process, apr_port_t port,
     }
 
     if (last == NULL) {
-        ap_listeners = last = new;
+        ap_listeners = new;
     }
     else {
         last->next = new;
-        last = new;
     }
 
     return NULL;