transaction as aborted.  Since we only call XactLockTableWait on XIDs
that we believe to be currently running, the odds of this code ever
actually firing are minimal.  It's certainly unnecessary, since a
transaction that's not either running or committed will be presumed
aborted anyway.  What's more, it's not hard to imagine scenarios where
this could result in corrupting pg_clog: for instance, if a bogus XID
somehow got passed to XactLockTableWait.  I think the code probably
dates from the ancient era when we didn't have TransactionIdIsInProgress;
back then it may have been necessary, but now I think it's a waste of
cycles and potentially dangerous.  Per discussion with Qingqing Zhou
and Karsten Hilbert.