chore(swingset): re-enable test of unrecognizable orphan cleanup

This clause was commented out because bug #3378 was still pending at
the time. I think we happened to fix that as a side-effect of adding
`syscall.abandonExport` for #4951 (in PR #4957), but didn't realize we
could re-enable the clause.

closes #3378

Author: warner

packages/SwingSet/test/test-gc-kernel.js

@@ -1167,7 +1167,7 @@ test('terminated vat', async t => {
 
   // TODO: we still fail to clean up the [0,0] kref: #3378 is about finding
   // somewhere to delete the .refcount key
-  // t.is(refcounts[doomedExport2Kref], undefined);
+  t.is(refcounts[doomedExport2Kref], undefined);
   t.falsy(owners[doomedExport2Kref]);
 });