C++: Fix typo - remove unbound and unused variable in downgrade script.

Author: sashabu

cpp/downgrades/25e365d1e8147df0f759b604f96eb4bffea48271/using_container.ql

@@ -8,7 +8,7 @@ class Element extends @element {
 
 from UsingEntry u, Element parent, int kind
 where
-  usings(u, target, _, kind) and
+  usings(u, _, _, kind) and
   using_container(parent, u) and
   kind != 3
 select parent, u