Remove overzealous assertion from PHJ.

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 <[email protected]>
Reported-by: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/3590249.1680971629@sss.pgh.pa.us