Remove bogus generator iterator dtor

Remove bogus generator iterator dtor

Fixes a use-after-free encountered in Symfony's SecurityBundle.
I don't have a reproducer for this, and believe the issue can only
occur if we leak an iterator (the leak is a separate issue).

We should not free the generator iterator here, because we do not
own it. The code that fetched the iterator is responsible for
releasing it. In the rare case where we do hit this code-path,
we cause a use-after-free.