TransactionIdIsActive() has long been unused

Hi,

While looking at Tomas' NUMA patchset I got curious what we use
TransactionIdIsActive() for (it's modified in the patchset). Turns out we
don't. And haven't for a long while, the last use was removed in

commit bb38fb0d43c
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date:   2014-05-15 16:37:50 +0300

    Fix race condition in preparing a transaction for two-phase commit.

Seems like we should just remove TransactionIdIsActive()?

Greetings,

Andres Freund