Remove overzealous assertion from PHJ.

We can't assert that we're the only process attached to a barrier after
BarrierArriveAndDetachExceptLast().  Although that'll be true almost
always, a late-starting parallel worker can attach very briefly (that
is, immediately detach after checking the phase) right at that moment.
BarrierArriveAndDetachExceptLast() already contains an assertion like
that, but it holds a spinlock preventing the race.  This thinko caused a
one-off failure on build farm animal chimaera.

Diagnosed-by: Melanie Plageman <melanieplageman@gmail.com>
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3590249.1680971629@sss.pgh.pa.us

src/backend/executor/nodeHash.c

@@ -2115,7 +2115,6 @@ ExecParallelPrepHashTableForUnmatched(HashJoinState *hjstate)
 
 	/* Now we are alone with this batch. */
 	Assert(BarrierPhase(&batch->batch_barrier) == PHJ_BATCH_SCAN);
-	Assert(BarrierParticipants(&batch->batch_barrier) == 1);
 
 	/*
 	 * Has another process decided to give up early and command all processes