No tree existence measures

Don't try hard to clean up trees that aren't transaction.

With Persistit (2.x), this never really got the job done due to https://bugs.launchpad.net/akiban-persistit/+bug/1055557 anyway. Note that they are left in an empty state as the keys and values are properly transactional.

Additionally remove isTreeRemovalDelayed as it is never false when running with Persistit and is no longer needed.

Nice bit of deletion for the alter and index creation path.