Optimize remove_level_2

To reset the identifier we can just used the l1_object_identifier which
is a cached version of the level1 identifier.

Author: romainbrenguier

src/util/ssa_expr.cpp

@@ -114,5 +114,5 @@ void ssa_exprt::update_identifier()
 void ssa_exprt::remove_level_2()
 {
   remove(ID_L2);
-  update_identifier();
+  set_identifier(get_l1_object_identifier());
 }