Remove a 9.6 merge fixme in FinishPreparedTransaction()
The comment explains the scenario. wait_for_mirror() is still needed.  For
example the case of commmit-prepared retry (SyncRepWaitForLSN() failure).
@@ -1409,8 +1409,6 @@ FinishPreparedTransaction(const char *gid, bool isCommit, bool raiseErrorIfNotFo
gxact = LockGXact(gid, GetUserId(), raiseErrorIfNotFound);
if (gxact == NULL)
{
- /* GPDB_96_MERGE_FIXME: understand this comment, and figure out if
- * this block is still needed. */
/*
* We can be here for commit-prepared and abort-prepared. Incase of
* commit-prepared not able to find the gxact clearly means we already
